New invitation backend for portfolio invitations.

Portfolio invitations do not associate a user entity until the
invitation has been accepted. User info, including DOD ID, is held on
the invitation itself. When a user accepts and invitation, their user
entry is associated with the corresponding `portfolio_role` entry.

The same change will be applied to `application_role` and application
invitations. For now, small changes have been made to
application-related methods so that that flow works as-is.

atst/models/mixins/invites.py

@@ -97,7 +97,7 @@ class InvitesMixin(object):
 
     @property
     def user_name(self):
-        return self.role.user.full_name
+        return "{} {}".format(self.first_name, self.last_name)
 
     @property
     def is_revokable(self):