* 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
This is required because "no interface" is the same as showing
the first entry. But when the top menu has changed the first
entry is no longer the same, so it shows the wrong default.
* Add user_home configuration option
* Use the new user_home preference to actually show the right home page
* Fix trailing whitespace
* Update user_option_serializer.rb
* Fix JavaScript default homepage tests
* Use an object instead of a giant switch
* Remove trailing whitespace
* Make the default `user_home` set to `null` instead of `0`
* Rename user_home to homepage_id
