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):

atst/models/portfolio_role.py

@@ -116,7 +116,14 @@ class PortfolioRole(
 
     @property
     def user_name(self):
-        return self.user.full_name
+        if self.user:
+            return self.user.full_name
+        else:
+            return self.latest_invitation.user_name
+
+    @property
+    def full_name(self):
+        return self.user_name
 
     @property
     def is_active(self):
@@ -128,10 +135,6 @@ class PortfolioRole(
             self.latest_invitation and self.latest_invitation.is_inactive
         )
 
-    @property
-    def full_name(self):
-        return self.user.full_name
-
     @property
     def application_id(self):
         return None