Hidden annotations with replies still shown after reply deletion

So I’m not entirely sure whether this counts as a bug – _technically_, this is working as designed – and I’m pretty sure it’s a very minor issue. Writing it up as an issue seemed like as good a way as any to deal with it, though.

## Steps to reproduce

1. As a member of a third-party group (or as a user for whom flagging is enabled), flag an annotation
2. As a member or the group’s creator, create a reply to the flagged annotation
3. As a moderator, hide the flagged annotation
4. As the reply author, delete the reply

As the behaviour currently stands, the hidden annotation will be displayed with its content blanked out, as we currently show for [hidden annotations with replies](https://github.com/hypothesis/product-backlog/issues/231), whereas if the reply had never been posted in the first place, it would be hidden entirely. It will then disappear entirely the next time we run a full search re-index.

The fact that this is inconsistent (that the way we display an annotation will change after a reindex) probably suggests this counts as a bug, but I suspect it’s going to be rare enough, and there’s an argument for the behaviour in either direction, that it’s not worth prioritising and we can close this issue as a `wontfix`.