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.
:+1: Noah explained this one to me - this is a template tag (used in just one place, right?) which performed a db lookup to get the primary context for the page.  This seems much more at home in a view (and I believe the function Noah replaced it with also caches).