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/users_controller.rb

@@ -1072,7 +1072,7 @@ class UsersController < ApplicationController
       :title,
       :date_of_birth,
       :muted_usernames,
-      :theme_key,
+      :theme_ids,
       :locale,
       :bio_raw,
       :location,
@@ -1087,7 +1087,7 @@ class UsersController < ApplicationController
     permitted.concat UserUpdater::TAG_NAMES.keys
 
     result = params
-      .permit(permitted)
+      .permit(permitted, theme_ids: [])
       .reverse_merge(
         ip_address: request.remote_ip,
         registration_ip_address: request.remote_ip,