#9 students can add images to assignments

[fRiSi]

students are allowed to add images to assignments.

since image has the ordinary ploneworkflow this is possible

possible solutions:
- take away add content permission
maybe they are not allowed to add their completed
assignments anymore

- own workflow for image
a lot of work, surely not the nicest way to do that

this is not critical because if a student creates an
image we can see that he is the owner and responsible
for this nonsense.

but if a student presses add image by mistake an object
gets created (todo: maybe it gets deleted if he presses
cancel)