FEATURE: Native theme support

This feature introduces the concept of themes. Themes are an evolution
of site customizations.

Themes introduce two very big conceptual changes:

- A theme may include other "child themes", children can include grand
children and so on.

- A theme may specify a color scheme

The change does away with the idea of "enabled" color schemes.

It also adds a bunch of big niceties like

- You can source a theme from a git repo

- History for themes is much improved

- You can only have a single enabled theme. Themes can be selected by
    users, if you opt for it.

On a technical level this change comes with a whole bunch of goodies

- All CSS is now compiled using a custom pipeline that uses libsass
    see /lib/stylesheet

- There is a single pipeline for css compilation (in the past we used
    one for customizations and another one for the rest of the app

- The stylesheet pipeline is now divorced of sprockets, there is no
   reliance on sprockets for CSS bundling

- CSS is generated with source maps everywhere (including themes) this
    makes debugging much easier

- Our "live reloader" is smarter and avoid a flash of unstyled content
   we run a file watcher in "puma" in dev so you no longer need to run
   rake autospec to watch for CSS changes

app/controllers/admin/themes_controller.rb

@@ -0,0 +1,192 @@
+class Admin::ThemesController < Admin::AdminController
+
+  skip_before_filter :check_xhr, only: [:show]
+
+  def import
+
+    @theme = nil
+    if params[:theme]
+      json = JSON::parse(params[:theme].read)
+      theme = json['theme']
+
+      @theme = Theme.new(name: theme["name"], user_id: current_user.id)
+      theme["theme_fields"]&.each do |field|
+        @theme.set_field(field["target"], field["name"], field["value"])
+      end
+
+      if @theme.save
+        log_theme_change(nil, @theme)
+        render json: @theme, status: :created
+      else
+        render json: @theme.errors, status: :unprocessable_entity
+      end
+    elsif params[:remote]
+      @theme = RemoteTheme.import_theme(params[:remote])
+      render json: @theme, status: :created
+    else
+      render json: @theme.errors, status: :unprocessable_entity
+    end
+
+  end
+
+  def index
+    @theme = Theme.order(:name).includes(:theme_fields, :remote_theme)
+    @color_schemes = ColorScheme.all.to_a
+    light = ColorScheme.new(name: I18n.t("color_schemes.default"))
+    @color_schemes.unshift(light)
+
+    payload = {
+      themes: ActiveModel::ArraySerializer.new(@theme, each_serializer: ThemeSerializer),
+      extras: {
+        color_schemes: ActiveModel::ArraySerializer.new(@color_schemes, each_serializer: ColorSchemeSerializer)
+      }
+    }
+
+    respond_to do |format|
+      format.json { render json: payload}
+    end
+  end
+
+  def create
+    @theme = Theme.new(name: theme_params[:name],
+                       user_id: current_user.id,
+                       user_selectable: theme_params[:user_selectable] || false,
+                       color_scheme_id: theme_params[:color_scheme_id])
+    set_fields
+
+    respond_to do |format|
+      if @theme.save
+        update_default_theme
+        log_theme_change(nil, @theme)
+        format.json { render json: @theme, status: :created}
+      else
+        format.json { render json: @theme.errors, status: :unprocessable_entity }
+      end
+    end
+  end
+
+  def update
+    @theme = Theme.find(params[:id])
+
+    original_json = ThemeSerializer.new(@theme, root: false).to_json
+
+    [:name, :color_scheme_id, :user_selectable].each do |field|
+      if theme_params.key?(field)
+        @theme.send("#{field}=", theme_params[field])
+      end
+    end
+
+    if theme_params.key?(:child_theme_ids)
+      expected = theme_params[:child_theme_ids].map(&:to_i)
+
+      @theme.child_theme_relation.to_a.each do |child|
+        if expected.include?(child.child_theme_id)
+          expected.reject!{|id| id == child.child_theme_id}
+        else
+          child.destroy
+        end
+      end
+
+      Theme.where(id: expected).each do |theme|
+        @theme.add_child_theme!(theme)
+      end
+
+    end
+
+    set_fields
+
+    if params[:theme][:remote_check]
+      @theme.remote_theme.update_remote_version
+      @theme.remote_theme.save!
+    end
+
+    if params[:theme][:remote_update]
+      @theme.remote_theme.update_from_remote
+      @theme.remote_theme.save!
+    end
+
+    respond_to do |format|
+      if @theme.save
+
+        update_default_theme
+
+        log_theme_change(original_json, @theme)
+        format.json { render json: @theme, status: :created}
+      else
+        format.json { render json: @theme.errors, status: :unprocessable_entity }
+      end
+    end
+  end
+
+  def destroy
+    @theme = Theme.find(params[:id])
+    StaffActionLogger.new(current_user).log_theme_destroy(@theme)
+    @theme.destroy
+
+    respond_to do |format|
+      format.json { head :no_content }
+    end
+  end
+
+  def show
+    @theme = Theme.find(params[:id])
+
+    respond_to do |format|
+      format.json do
+        check_xhr
+        render json: ThemeSerializer.new(@theme)
+      end
+
+      format.any(:html, :text) do
+        raise RenderEmpty.new if request.xhr?
+
+        response.headers['Content-Disposition'] = "attachment; filename=#{@theme.name.parameterize}.dcstyle.json"
+        response.sending_file = true
+        render json: ThemeSerializer.new(@theme)
+      end
+    end
+
+  end
+
+  private
+
+    def update_default_theme
+      if theme_params.key?(:default)
+        is_default = theme_params[:default]
+        if @theme.key == SiteSetting.default_theme_key && !is_default
+          Theme.clear_default!
+        elsif is_default
+          @theme.set_default!
+        end
+      end
+    end
+
+    def theme_params
+      @theme_params ||=
+        begin
+          # deep munge is a train wreck, work around it for now
+          params[:theme][:child_theme_ids] ||= [] if params[:theme].key?(:child_theme_ids)
+          params.require(:theme)
+            .permit(:name,
+                    :color_scheme_id,
+                    :default,
+                    :user_selectable,
+                    theme_fields: [:name, :target, :value],
+                    child_theme_ids: [])
+        end
+    end
+
+    def set_fields
+
+      return unless fields = theme_params[:theme_fields]
+
+      fields.each do |field|
+        @theme.set_field(field[:target], field[:name], field[:value])
+      end
+    end
+
+    def log_theme_change(old_record, new_record)
+      StaffActionLogger.new(current_user).log_theme_change(old_record, new_record)
+    end
+
+end