FEATURE: Polymorphic bookmarks pt. 3 (reminders, imports, exports, refactors) (#16591)

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.

plugins/discourse-narrative-bot/lib/discourse_narrative_bot/new_user_narrative.rb

@@ -243,6 +243,7 @@ module DiscourseNarrativeBot
       post
     end
 
+    # TODO (martin) [POLYBOOK] Fix up narrative bot bookmark interactions in a separate PR.
     def missing_bookmark
       return unless valid_topic?(@post.topic_id)
       return if @post.user_id == self.discobot_user.id
@@ -253,6 +254,7 @@ module DiscourseNarrativeBot
       false
     end
 
+    # TODO (martin) [POLYBOOK] Fix up narrative bot bookmark interactions in a separate PR.
     def reply_to_bookmark
       return unless valid_topic?(@post.topic_id)
       return unless @post.user_id == self.discobot_user.id