When we send a bookmark reminder, there is an option to delete
the underlying bookmark. The Notification record stays around.
However, if you want to filter your notifications user menu
to only bookmark-based notifications, we were not showing unread
bookmark notifications for deleted bookmarks.
This commit fixes the issue _going forward_ by adding the
bookmarkable_id and bookmarkable_type to the Notification data,
so we can look up the underlying Post/Topic/Chat::Message
for a deleted bookmark and check user access in this way. Then,
it doesn't matter if the bookmark was deleted.
This is so we can join the Notification table onto the
Bookmark table. A slight refactor was needed to ensure
that the required values are always included and the
consumer does not need to think about this.
The discourse-chat and discourse-data-explorer plugins
will be updated to take advantage of this commit.