Regex support for checking usernames against the USERNAME_BLACKLIST.

A case-insensitive regex check to prevents users from creating Usernames that use *words* in the ACCOUNT_USERNAME_BLACKLIST.

if ACCOUNT_USERNAME_BLACKLIST = ['shit', 'ass',] 

then ......usernames like 'AssHat' and 'ShitMonkey' will be disallowed. This was a TODO item.

NOTE: While 'AssHat' would not be allowed; 'BassFisherman' and 'Lassie' would also get stopped! it might make sense to add a BLACKLIST_EXACT_MATCH option and then update the docs so that people can choose between an exact case-insensitve match (using re.match) or the more aggressive match on partial word approach that I have used.