Use postgres:9.6 is ci

Nearly completed typing the whole store!

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.

Feedback is now filterable by tutor (reviewer only)

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.

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