Implemented + add the option "mandatory whenever possible".

So we now have:
- optional (allow empty values)
- mandatory whenever possible (mandatory as it were originally)
- mandatory only if it was presented to the original submitter (cern relaxed mandatory)

I first thought about allowing to configure the mandatory per authentication level. It would have lead to the same functionalities but with senseless and confusing choice: only members can in fact do anything beyond submitting items.