Hide splits

Implements #19 (closed)

This branch is based on add-split-name as it needs the edit_cut endpoint. Splits have an additional hidden property that results in the client not rendering that part of the exam. If the pdf-section is a non-cut section (the last section of a page) a new cut with hidden = True is created.

Possible improvements: Better visualization for admins wether a part of the pdf is visible