These validate/after_create/after_destroy methods were added
back in b8828d4a2d before
the RegisteredBookmarkable API and pattern was nailed down.
This commit updates BookmarkManager to call out to the
relevant bookmarkable for these and bookmark_metadata for
consistency.
A bit of a mixed bag, this addresses several edge areas of bookmarks and makes them compatible with polymorphic bookmarks (hidden behind the `use_polymorphic_bookmarks` site setting). The main ones are:
* ExportUserArchive compatibility
* SyncTopicUserBookmarked job compatibility
* Sending different notifications for the bookmark reminders based on the bookmarkable type
* Import scripts compatibility
* BookmarkReminderNotificationHandler compatibility
This PR also refactors the `register_bookmarkable` API so it accepts a class descended from a `BaseBookmarkable` class instead. This was done because we kept having to add more and more lambdas/properties inline and it was very messy, so a factory pattern is cleaner. The classes can be tested independently as well.
Some later PRs will address some other areas like the discourse narrative bot, advanced search, reports, and the .ics endpoint for bookmarks.
