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.

A map file generated by hektor can be loaded into the app and is then used to map the obfuscated names and matrikel numbers back