First shot on custom user management (#1)

Ref: https://github.com/ideas-box/ideasbox.lan/issues/1
I may be missing a point but, here is a bit of explanations:
- every Box context (UNHCR boxes in Burundi, PNUD box in Burundi, UNHCR boxes in Jordan, etc.) will have it's specific user class, which will compose will all the available mixins, and maybe have some specific fields.
- AbstractUser is `abstract`, because we don't want model inheriting from it to have a fk to it
- DefaultUser is the minimal non abstract model to be used for dev and tests

Makes sense? :)