New backend flow for application invitations.

Like portfolio invitations, now a user is not associated with an
application role until they accept the associated invitation.
- domain method for inviting user to application
- change application route for inviting a member
- ApplicationRole model knows user name from invitation

atst/models/application_role.py

@@ -59,12 +59,17 @@ class ApplicationRole(
         ),
     )
 
+    @property
+    def latest_invitation(self):
+        if self.invitations:
+            return self.invitations[-1]
+
     @property
     def user_name(self):
         if self.user:
             return self.user.full_name
-        else:
-            return None
+        elif self.latest_invitation:
+            return self.latest_invitation.user_name
 
     def __repr__(self):
         return "<ApplicationRole(application='{}', user_id='{}', id='{}', permissions={})>".format(