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.
df4197c8b8