Use inbox rels to determine if a thing is valid cause for blocking.

This way we don't have to duplicate the logic that made the inbox rels
get created in the first place, but rather can just see if they do
exist.
Gold Feature: "The Butler". Username monitoring in comments.

This is a new feature available only to Gold members. It defaults to on,
but may be turned off in preferences. When a username is mentioned on
the site in the /u/username format, and some conditions apply, the user
is notified by an orangered and a new type of item in the inbox. There
are several cases where notifications won't occur to reduce noise, see
butler.py for details.