Notifications on Things that have been deleted are problematic

(Bug, Investigate , Priority: Medium, Test Status: No automated tests yet , Reported By Bad Link: Thing 3y284oe not found, )

Conversations, Notifications

Summary: This isn't solely related to Comments, but those are the sharp end of the problem.

Say that I get a Comment about a Thing. Then that Thing gets deleted. What happens?

Currently, the Notification sticks around, but clicking on it goes to a dead page. The Comment is probably unreachable.

What should happen? Not obvious. Trying to remove the Notification would be sort of nice, but is likely to be computationally infeasible -- we're not indexed to do that, nor do we want to be, since it would be extremely expensive. But it's worth thinking about.