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/models/user_option.rb

@@ -1,4 +1,7 @@
 class UserOption < ActiveRecord::Base
+  # TODO: remove in 2019
+  self.ignored_columns = ["theme_key"]
+
   self.primary_key = :user_id
   belongs_to :user
   before_create :set_defaults
@@ -177,10 +180,10 @@ end
 #  mailing_list_mode_frequency      :integer          default(1), not null
 #  include_tl0_in_digests           :boolean          default(FALSE)
 #  notification_level_when_replying :integer
-#  theme_key                        :string
 #  theme_key_seq                    :integer          default(0), not null
 #  allow_private_messages           :boolean          default(TRUE), not null
 #  homepage_id                      :integer
+#  theme_ids                        :integer          default([]), not null, is an Array
 #
 # Indexes
 #