Blacklist for usernames, display names and collection names (bug 1024630)

WiP: I still need to test the admin part.
ah, you got a point. I don't really care which app it's in, as long as there's no "blacklist" app... and I don't think we need a blacklist app. This leaves us with two choices:
1/ leave this in the users app
2/ move it to "core" or "utils" or some place like that

Your call ;)