Add permissions

[bouttier]

We should move from staff status to more complexe permissions managment system.
At least, we should consider admin (right to break everythings, including email address, url, etc), staff (non destructive operations) and reviewers (see talks and vote).
They are several way to implement this:

• one ManyToManyFields per role on Conference model
• a Permission model (conf, user, role)
  The second method is probably better to add future new roles.