Unify the invitation UI

(Bug, Investigate , Priority: High, Test Status: No automated tests yet , Reported By Justin du Coeur, )
UX, Invites
Summary: Currently, the Sharing Page has separate boxes for inviting by email vs. inviting collaborators. That's silly -- it should be one box, and we should recognize which is which.
I'm calling the current state a UX bug.
The UI should enforce that any entry is either an email address or a known collaborator, and refuse to accept it otherwise. It should provide an appropriate error message if it is neither.