It was a dropdown to provide choices of color schemes,
and only one scheme could be shown.
With this commit, multiple color scheme previews can be displayed on
one page at the same time, making admins choose color schemes more
easily.
Theme preview windows are shrinked.
Imported default color schemes.
Co-Authored-By: Misaka 0x4e21 <misaka4e21@gmail.com>
* 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
