[DEV-1699] Remove longerusername (more monkey-patching).

This ultimately inherits from forms.CharField, which doesn't have a default max_length.  It's meant for entry of a username for auth_user, which has a maximum length of 30.

We don't actually use this field anywhere.  It gets used in Askbot's forked django_authopenid, and in Askbot if users are allowed to edit their usernames, which I'm going to turn off in a second.