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, #53.

* The mechanism remains partially incomplete as the progress in
  !67-create-new-model-feedbackline-and-integrate-it is blocking
  progress. Several tests for the API endpoint need to be written
  including. More validation and constraints might have to be added.