Also changed the length restriction of the StudentInfo.matrikel_no to 30

The reviewer has the option to activate and deactivate all students access via the web interface in the student overview. The corresponding endpoints are additional list routes on the student viewset. Tests are in test_reviewer_viewset.py

closes #91

Only Comments that are `visible_to_student=True` will be serialized. For some weird reason i had to resort to a little hack in the serializer, see the comment inside the `get_feedback_lines()` method of the serializer for context.
I choose to not remove Feedback that is not final from the response of the student submissions endpoint (as outlined in #91) and will instead show a message in the frontend. This is easier to implement and potentially better for debugging in the frontend.

Forgot one slash...

Uniform naming makes Frontend life easier

The issue was that
```python
	 response = client.post(
            f'/api/feedback/', {
                "score": 23,
                "of_submission": response.data['submission']['pk'],
                "feedback_lines": {
                    2: {"text": "< some string >"},
                    3: {"text": "< some string >"}
                }
            }
        )
        self.assertEqual(status.HTTP_201_CREATED, response.status_code)
```
would sometimes get the one submission that only had 2 lines (since submission distribution hasn't been randomized for long, this issue didn't come up earlier) and thus fail as it should.
I changed the commented line to 1 and 2 in the POST. This should fix the issue.

Fixed subscription deletion

Added welcome Jumbotron with some nice info/links

Some work on start page

Added feedback_stage_for_user to feedback serializer

This field will contain the feedback stage of the subscription for which the requesting user has created this feedback. This is needed to correctly give the choice to set the final attribute in the frontend.

Fix final checkbox frontend

Fix disappearing names in feedback list

Solution Syntax highlighting

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

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

Implemented Feedback History Table incl. search

Side Note: When searching the table, should the resulting filtered arr be <= than size 10 all those submissions will be prefetched if not already present and the user is able to instantanously view the submission. NICE!

Added Tests and sticky beahvior for feedback hist

Inactive subscriptions are shown

Added subscription refresh button

Fixed jwt refresh and auto logout

Added feedback link

Visual cue of final status in student overview

Correct Student for reviewer

* Reviewer feedback cannot be edited by tutors (despite assignments)
* Tutors can always edit feedback for which they have an assignment
* Reviewer is allowed to change anything anytime