For the following conditions, the TopicUser.bookmarked column was not updated correctly:
* When a bookmark was auto-deleted because the reminder was sent
* When a bookmark was auto-deleted because the owner of the bookmark replied to the topic
This adds another migration to fix the out-of-sync column and also some refactors to BookmarkManager to allow for more of these delete cases. BookmarkManager is used instead of directly destroying the bookmark in PostCreator and BookmarkReminderNotificationHandler.