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

db/fixtures/000_delayed_drops.rb

@@ -217,4 +217,26 @@ Migration::TableDropper.delayed_drop(
   }
 )
 
+Migration::ColumnDropper.drop(
+  table: 'user_options',
+  after_migration: 'DropKeyColumnFromThemes',
+  columns: %w[
+    theme_key
+  ],
+  on_drop: ->() {
+    STDERR.puts 'Removing theme_key column from user_options table!'
+  }
+)
+
+Migration::ColumnDropper.drop(
+  table: 'themes',
+  after_migration: 'DropKeyColumnFromThemes',
+  columns: %w[
+    key
+  ],
+  on_drop: ->() {
+    STDERR.puts 'Removing key column from themes table!'
+  }
+)
+
 Discourse.reset_active_record_cache