Show invitations on project dashboard

These changes address this ticket: http://manage.dimagi.com/default.asp?115952

This PR adds a list of pending domain invitations to the domain select page (http://0.0.0.0:8000/domain/select/) with accept buttons next to each one. It also moves some of the logic for getting a users projects from a template tag and into the appropriate django view. I also created a new couch view so that I could get all invitations for a given user.
I didn't know about that.  Definitely sounds like a good option.  @snopoke Invitations would probably be one of the easier ones to convert to SQL - they're relatively small and simple, and losing data in the migration is much less risky.