Draft: subject drafts

This adds the new SubjectDraft model which can either be converted into a "real" subject or discarded. In accordance with the database split, contact data is stored in a separate ContactDraft model.

The wording is still far from final.

I did not add a way to merge a SubjectDraft into an existing Subject. I don't think this should be solved on a technical level alone. It is not clear what would be the "right" way to do it. But right now users cannot even see the recruitment attributes included with the SubjectDraft. Dynamic attributes don't make this any easier.