Use postgres:9.6 is ci

The student page will no longer crash should there be no feedback objects for submissions and display a message to the user explaining that the submission hasn't been corrected due to the exam beeing pass only.

Closes #101

Added HIDE/SHOW FEEDBACK Button in submission Correction top toolbar to toggle the visibilty of all comments (own as well as others). Open editors are not closed.

closes #91

Forgot one slash...

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

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

Impplemented skip functionality when correcting

Subscriptions are now deletable

Added tqdm to requierements

Changed jwt auth endpoints

`/api-token-auth/` to `/api/get-token/`
`/api-token-refresh/` to `/api/refresh-token/`

Added student list page

Added submission serializer for student list view

Bare student list view is now implemented for the reviewer
He is able to view a searchable and sortable list of all students in the left pane of the window and their submissions on the right side

Fixed /?#/ bug on login page

Added `feedback_created`/`feedback_validated` counts to TutorSerializer

Added basic tutor overview

Added student overview help card

When going to the student overview page, a help card is shown on the right side which is replaced when viewing a submission