Started with a new mechanism to assign work to tutors

• The mechanism proposed should work as follows:

  • Tutors can subscribe to certain submission categries (currently this includes exam, student or type specific submissions). If the set of submissions to corrent is small (student) all submissions of that category are reserved for that tutor.
  • A reviewer should also be able to subscribe other users (delegation)
  • A subscription contains assignments or creates them:
    • Only one assignment per user may be active.
    • No new assignments can be added to a subscription after it was created while another assignment is present for that subscription.
    • An assignment delegates a submission to a tutor.
    • An active assignment indicates that the tutor is working on that assignment
    • After an assignment was finished it is deleted (or archived).

• Upgraded to Django 2.0

• Closes #66 (closed), #53 (closed).