First part for multidelete

[vmcj]

This adds the functionality to delete multiple: contests problems

It can be extended to also do other types of entities but I'm not sure if it makes sense for everything and there are some pages that don't have the primaryKey rendered in the correct part. This makes extending need a rewrite. I prefer to first fix the easy things and extend on this in the future when this passes codereview.

This might also close https://github.com/DOMjudge/domjudge/issues/229 if we agree that no more endpoints are needed.