First shot on custom user management (#1)

Ref: https://github.com/ideas-box/ideasbox.lan/issues/1
We need to have all the dependencies installable through apt, so I'm a bit in the mantra "the less deps, the saner", but I take your point (already thought about it actually, because we had a django-choices too in Libé as you may remember).

So what I suggest for now is to keep it as is, and if in the first month (before releasing the first Ideas Box running this code) to see if the need around choices grow. Sounds fair? :)