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

spec/requests/admin/themes_controller_spec.rb

@@ -144,25 +144,25 @@ describe Admin::ThemesController do
     let(:theme) { Theme.create(name: 'my name', user_id: -1) }
 
     it 'can change default theme' do
-      SiteSetting.default_theme_key = nil
+      SiteSetting.default_theme_id = -1
 
       put "/admin/themes/#{theme.id}.json", params: {
         id: theme.id, theme: { default: true }
       }
 
       expect(response.status).to eq(200)
-      expect(SiteSetting.default_theme_key).to eq(theme.key)
+      expect(SiteSetting.default_theme_id).to eq(theme.id)
     end
 
     it 'can unset default theme' do
-      SiteSetting.default_theme_key = theme.key
+      SiteSetting.default_theme_id = theme.id
 
       put "/admin/themes/#{theme.id}.json", params: {
         theme: { default: false }
       }
 
       expect(response.status).to eq(200)
-      expect(SiteSetting.default_theme_key).to be_blank
+      expect(SiteSetting.default_theme_id).to eq(-1)
     end
 
     it 'updates a theme' do