Move favorite forums to user defaults.

Now the Core Data store contains absolutely no user data. It is entirely a
cache. It can be deleted with impunity. (Which has always been a possibility,
given it's in the Caches folder.)

The custom Core Data migration exists purely to extract favorite forums info.
Synchronous settings notifications on main thread.

Makes it easier to ignore the notifications when the user interface is already
up-to-date.