Add support for adding multiple members to a group at once

Adding multiple members at once is done via PATCH on
`groups/{group-oid}` with a body containing a list of refernces to the
members.

graph/pkg/service/v0/logging.go

@@ -69,6 +69,11 @@ func (l logging) PostGroup(w http.ResponseWriter, r *http.Request) {
 	l.next.PostGroup(w, r)
 }
 
+// PatchGroup implements the Service interface.
+func (l logging) PatchGroup(w http.ResponseWriter, r *http.Request) {
+	l.next.PatchGroup(w, r)
+}
+
 // DeleteGroup implements the Service interface.
 func (l logging) DeleteGroup(w http.ResponseWriter, r *http.Request) {
 	l.next.DeleteGroup(w, r)