* Defnied more APIendpoints for feedback and subscriptions
 * Added the feedback spec to the docs folder
 * The previous commit also closes #43 and #68

* 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.
* This code was written with the assumption that the user model
  will hold group information in the future.

* on creation or deletation of an assignment of the subscription
  a new submission is looked up. Therefore, currently deleting an
  assignment may leads to SubscriptionEnded exception.
* Cleaned up the urls config

* 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.

All the methods in core.tests.data_factories have been deleted and the tests using those are now using the improved make_test_data method from util.factories.
Curiously the tests are running considerably faster on my machine now

Closes #29.

* also moved test concerning access rights to another test suite