Allow creation of users programatically

There have been requests in past to allow pre-creation or users and disabling of "Sign up" button in Mailman when people want to pre-provision the users. Currently there is no easy way to do that, although, it can be made easier using a script that can simply do User.objects.create() or something taking a list of username, name, email, password as input csv.

We could also add an API for that purpose, but then we'd need some sort of API authentication mechanism.