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.

tests/domain/test_application_roles.py

@@ -33,7 +33,7 @@ def test_enabled_application_role():
     )
     assert app_role.status == ApplicationRoleStatus.DISABLED
 
-    ApplicationRoles.enable(app_role)
+    ApplicationRoles.enable(app_role, app_role.user)
 
     assert app_role.status == ApplicationRoleStatus.ACTIVE