Revise permissions

this MR revises the permission system and implements the permissions that we discussed during the last meeting. The following permissions are set:

• edit academic organization contact: only the contact itself (or the contact of the super organization)
• edit topic leader: only the leader himself, or a contact of the lead organization hierarchy
• approve topic memberships: same as edit topic leader
• end topic membership: same as topic leader, plus the member himself

I should write some text about how we handle the permissions and the updates (as this is managed via django-guardian).

closes #64 (closed), #63 (closed), #57 (closed), #56 (closed)