We should let folks define a list of banned user IDs and displayName regexps (SYN-729)

Submitted by @matthew:matrix.org
So that if an admin doesn't want people on their server using 'bad' words in their nicks/displaynames they can enforce it. This would also then reduce the amount of problems with bridged networks then rejecting 'bad' nicks and displaynames as per matrix-org/matrix-appservice-irc#103