It makes more sense to use user_ids for the UserCommScreener
introduced in fa5f3e228c since
in most cases the ID will be available, not the username. This
was discovered while starting work on a plugin that will
use this. In the cases where only usernames are available
the extra query is negligble.
The idea behind this refactor is to centralise all of the user ignoring / muting / disallow PM checks in a single place, so they can be used consistently in core as well as for plugins like chat, while improving the main bulk of the checks to run in a single fast non-AR query.
Also fixed up the invite error when someone is muting/ignoring the user that is trying to invite them to the topic.
