Student names in Subscription list for reviewer

Short polling on orig Feedback

Refresh button student list

FeedbackComment visiblity indication

SubscriptionEnded page

Right side of student overview is sticky

Statistics overview

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'