Add support for logging in with nicknames

[Morgul]

Right now, we require a login with [email protected]. It would be nice if we enabled logins with nick names, since that's less to type.

The question, though, is how do we implement this? Do we want nicknames to be required to be unique? Do we allow them to be changed?

At the moment, I think we force nicknames to be unique, and we do allow them to be changed, but at the time of the change, it needs to still be unique.

/cc @lordnull, @Burstaholic, @whitelynx I would love your thoughts on this.