Remove old Core Data model versions.

Core Data's finally, officially just a cache, and I don't care if it gets
migrated anymore.
Stop using mogenerator.

I'll just update the classes myself.
Remove old Core Data migration that extracted favorites into user defaults.

I don't care about anyone who hasn't yet updated to the point where this is
fixed.
Clean up and make explicit use of managed object context.

- Remove migration code from AwfulDataStack.
- Explicitly pass managed object context (or an object therefrom) to view
  controllers, managed objects, and other objects.
- Fix AwfulForumListController's state preservation.
- Directly save the relevant managed object context.
- Rename managed object model and explicitly refer to it.
- Move Core Data models and related code to "Data" folder.
- Remove data stack will/did reset notifications. If a replacement is needed,
  use NSPersistentStoreCoordinatorStoresWillChangeNotification and check if any
  stores will remain.