FEATURE: Groundwork for user-selectable theme components

* Phase 0 for user-selectable theme components

- Drops `key` column from the `themes` table
- Drops `theme_key` column from the `user_options` table
- Adds `theme_ids` (array of ints default []) column to the `user_options` table and migrates data from `theme_key` to the new column.
- Removes the `default_theme_key` site setting and adds `default_theme_id` instead.
- Replaces `theme_key` cookie with a new one called `theme_ids`
- no longer need Theme.settings_for_client

app/controllers/admin/themes_controller.rb

@@ -7,7 +7,7 @@ class Admin::ThemesController < Admin::AdminController
 
   def preview
     @theme = Theme.find(params[:id])
-    redirect_to path("/?preview_theme_key=#{@theme.key}")
+    redirect_to path("/?preview_theme_id=#{@theme.id}")
   end
 
   def upload_asset
@@ -226,7 +226,7 @@ class Admin::ThemesController < Admin::AdminController
   def update_default_theme
     if theme_params.key?(:default)
       is_default = theme_params[:default].to_s == "true"
-      if @theme.key == SiteSetting.default_theme_key && !is_default
+      if @theme.id == SiteSetting.default_theme_id && !is_default
         Theme.clear_default!
       elsif is_default
         @theme.set_default!