Set default theme for forums with their own themes.

* Defaults go into the hidden section of Settings.plist.
* Attempting to set a forum's theme to its default will instead clear the saved
  value from the user's preferences.
* No need to pick a default YOSPOS theme when migrating old settings.
* Bonus: add a description to the BYOB theme.
* Bonus: some formatting in AwfulSettings.m.
* Bonus: DRY up deriving form-specific settings keys.