Using multiprocessing.Lock to synchronize access to creation of Assignment

fixes #130

* This means a 2x - 10x performace boost especially for the student
  and the feedback endpoint

Impplemented skip functionality when correcting

Subscriptions are now deletable

Added tqdm to requierements

* Also made minor modifications to assignment endpoint

* 'submission_pk' -> 'submission' on AssignmentSerializer
* subscription now uniformly use the private key of a model
  that they want to receive submissions from
* introduced remaining and available fields on subscription
* query key and type are now checked

* It is now possible to 'deactivate subscriptions via the delete
  http verb
* This is not exactly what was specified in #92 but should achieve
  the same result. Instead of introducing a depleted field,
  subscriptions can distinguish if they are fully depleted or just
  temporarily. The method does not involve any overhead.
* Refactorings in the subscription model to increase readability
* Creating a subscription does not have side effects (creates no
  assignment)

Other minor changes

* Assignments are now implicitly checked if feedback is created
* using the assignment endpoint to create subscriptions instead of
  subscription endpoint

Closes #93 and #92.

* Added more tests for feedback view

* Now each view is included in one file that are held in the
  package views instead of using one big file

* Did the same this for serializers

* Now using ListSerializer to implement the custom behaviour

* Also refactored the feedback serializers and models and
  removed the FeedbackLine model

* The serializers are simpler now

* Renamed 'is_final' on FeedbackComment to 'visible_to_student'