This commit migrates all bookmarks to be polymorphic (using the
bookmarkable_id and bookmarkable_type) columns. It also deletes
all the old code guarded behind the use_polymorphic_bookmarks setting
and changes that setting to true for all sites and by default for
the sake of plugins.
No data is deleted in the migrations, the old post_id and for_topic
columns for bookmarks will be dropped later on.
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.
