Better identity and access management

[Bozydarek]

Right now users can be promoted to admin (staff) or superadmin (django-admin).
It's not the best solution, because all people promoted to staff, get access to some information and options that should be accessed only by a small subset of all organizers.

Probably we need more roles or system that will allow us to define groups/roles and their permissions.

This topic will become even more important when we will add more features for staff like for example #19.