Notify about mentions by name

[rossjrw]

The service could notify users when their name is mentioned in forum posts.

First suggested here by Kufat: https://scp-wiki.wikidot.com/forum/t-14209085/forum-notifications-service#post-5094544

This would be a new type of configuration. The UI would probably be a separate checkbox on the user config page.

Unsubscribed threads should be respected where possible.

Not sure what the correct approach is re grandfathering in existing users. I forsee a possible attack vector where a malicious user can ping lots of subscribed users to spam them - in fact this does already happen on e.g. IRC. It might end up being the case that this is a poor idea.