Return correct link for effective group roles in GET /role_assignments

The assignment link returned for roles that are included by virtue of
group membership should refer to the group assignment that led to this
role, rather than a direct user assignment.

Fixes bug 1201374

Change-Id: Ic649e7eb4633e258264f27280d938a08af380921
Merge "Return correct link for effective group roles in GET /role_assignments"