Support multiple groups

[krusche]

• There should be a concept of (research) groups that have their own advertised topics. Members of the group automatically create topics within the group. Currently, every advisor / supervisor can see all advertised topics and can also edit them
• When reviewing applications, users can only see the topics in the group unless they are super admin. However, users can currently accept or reject all applications, so this would also require permissions per group
• Currently admins see all theses and applications. There should be something like a group admin, which limits these permissions to the theses the at the group. The current admin should still be available for maintenance reasons, but should become a super admin
• When sending out emails, this should only notify people at the same group (presentations, new application etc)
• Add filter (e.g. selecting a specific group or professor as supervisor) and search options for topics in student applications and the front page
• More complex changes: Each group will have slightly different requirements as to what fields they want in the application. Currently, the fields are rather tailored to a coding heavy group. It should be configurable which data is requested per group in the application
• Each group should have the possibility to write custom email templates