Remove ProcessMode Repository

closes #1706 (closed)

(1) I made the corpus created in the migration to house the ex-Repository process public to avoid rights issues, but that's probably not ideal as it's a weird little fake corpus and it being public is strange. Maybe the better thing to do would be to give every existing user Guest access on it, and then also give Guest access to it to the default group to which all new users are added by default, if that's still a thing? I don't really remember how / if that works.

(2) I think the migrations work but I'm not 100% sure; they worked for my local DB with Repository processes in it, and they also work for the test DB which is being created every time, so it looks like they do?

I had trouble handling the fact that ProcessMode.Repository is present in a constraint on Process: at first I had just put a migrations.RemoveConstraint but that didn't work because the constraint is there in the inital process migrations, which failed when the ProcessMode enum was changed to remove Repository. Then I just removed the constraint from the initial migrations, as in the new ones I replace it with another one (either the process is Local or it has a corpus).

But I am Afraid it only looks like it works because of how I had initially removed the constraint and otherwise I'd be getting an error telling me a constraint with that name already exists.