Allow deletion of pictures and set pictures to private

[Nierot]

Right now, there is no easy way for the media to delete a picture they have uploaded mistakingly, nor can they set a public photo to private. I think there should be a button, only visible to the board (and maybe the media chair/secretary), that can delete a photo and another button that can change it's public status.

[supertom01]

For this issue we would create a button that alters the visibility (public & private) and a delete button (with confirmation), that is visible for the board and for the creator of the picture (since the processing of a chair function is rather difficult).

The delete and visibility buttons can be posited on the page of a picture next to the 'all pictures/alle fotos'