FEATURE: Add weekly bookmark cleanup code (PR #10899)

When posts or topics are deleted we don’t want to immediately delete associated bookmarks, so we have a grace period to recover them and their reminders if the post or topic is un-deleted. This PR adds a task to the Weekly scheduled job to go and delete bookmarks attached to posts or topics deleted > 3 days ago.


This could be in a site setting but I didn’t really see the value in making this configurable.

Looks great!