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/serializers/theme_serializer.rb

@@ -0,0 +1,42 @@
+class ThemeFieldSerializer < ApplicationSerializer
+  attributes :name, :target, :value
+
+  def target
+    case object.target
+    when 0 then "common"
+    when 1 then "desktop"
+    when 2 then "mobile"
+    end
+  end
+end
+
+class ChildThemeSerializer < ApplicationSerializer
+  attributes :id, :name, :key, :created_at, :updated_at, :default
+
+  def include_default?
+    object.key == SiteSetting.default_theme_key
+  end
+
+  def default
+    true
+  end
+end
+
+class RemoteThemeSerializer < ApplicationSerializer
+  attributes :id, :remote_url, :remote_version, :local_version, :about_url,
+             :license_url, :commits_behind, :remote_updated_at, :updated_at
+
+  # wow, AMS has some pretty nutty logic where it tries to find the path here
+  # from action dispatch, tell it not to
+  def about_url
+    object.about_url
+  end
+end
+
+class ThemeSerializer < ChildThemeSerializer
+  attributes :color_scheme, :color_scheme_id, :user_selectable, :remote_theme_id
+
+  has_many :theme_fields, serializer: ThemeFieldSerializer, embed: :objects
+  has_many :child_themes, serializer: ChildThemeSerializer, embed: :objects
+  has_one :remote_theme, serializer: RemoteThemeSerializer, embed: :objects
+end