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.
For next time - I think there is very little reason to add a new view to the `users` design doc versus adding a `couchapp` that just includes this view. we should probably just make this a ~hard rule moving forward to avoid massive reindexes of our large documents (unless we're deleting or modifying stuff). @dimagi/team-commcare-hq as an fyi or to agree/disagree.