The two Invitation fields should be merged

(Bug, To be Fixed , Priority: High, Test Status: No automated tests yet , Reported By Justin du Coeur, )
UX, Invites
Summary: Currently, the Sharing page has separate fields for Invite by Email and Invite by Collaborator. That's nicely easy, but sucks from a UX perspective.
The tricky bit here is going to be validation, and it may not be possible until we build our own Manifest implementation. As the user types, we should try to fetch collaborators, but it should be totally okay to not find them. Each selection must be either a found collaborator or a valid email address, though.