Pdf upload

Solves #148 (closed)

File upload question type

The new question type upload is added, which allows teachers to create a file upload question. Currently, only .zip and .pdf can be chosen as filetype (this can be done per question! So it is possible to have multiple upload questions with different file extension requirements).

Additionally solves some front-end issues: Solves #125 (closed) - move TA approval buttons to bottom side Solves #119 (closed) - disallow too big of a length (90k) chars for the open questions Solves #101 (closed) - disables editing of the rubric (now readonly) when peer review publish date has already expired