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

WiP: I still need to test the admin part.
The other option is to move it to the bandwagon app but I'm not sure that's worth it given that we clean 2 fields from the users app and one from the bandwagon one… I prefer to leave it as is, at least for now. We'll see if it eventually requires more logic as described in the ticket we may reconsider moving to its own dedicated app ;)