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/components/middleware/anonymous_cache_spec.rb

@@ -34,12 +34,12 @@ describe Middleware::AnonymousCache::Helper do
     it "handles theme keys" do
       theme = Theme.create(name: "test", user_id: -1, user_selectable: true)
 
-      with_bad_theme_key = new_helper("HTTP_COOKIE" => "theme_key=abc").cache_key
+      with_bad_theme_key = new_helper("HTTP_COOKIE" => "theme_ids=abc").cache_key
       with_no_theme_key = new_helper().cache_key
 
       expect(with_bad_theme_key).to eq(with_no_theme_key)
 
-      with_good_theme_key = new_helper("HTTP_COOKIE" => "theme_key=#{theme.key}").cache_key
+      with_good_theme_key = new_helper("HTTP_COOKIE" => "theme_ids=#{theme.id}").cache_key
 
       expect(with_good_theme_key).not_to eq(with_no_theme_key)
     end