Collections: Allow Editors to Manage editors?

[cycomachead]

Right now, only the owner of a collection can add/remove editors. given the way we designate '@snapcloud' as the owner, it'd be nice if we could manage editors, too.

(Maybe admins should just be able to do this?)

[cycomachead]

We need to table this since collections can be used for class projects. We perhaps want an admin role, but that's for later.