cmd/stcrashreceiver: Enable (rough) affected users count (#6511)

Seeing thousands of reports is no use when we don't know if they
represent one poor user or thousands.

cmd/stcrashreceiver/sentry.go

@@ -31,12 +31,14 @@ var (
 	clientsMut sync.Mutex
 )
 
-func sendReport(dsn, path string, report []byte) error {
+func sendReport(dsn, path string, report []byte, userID string) error {
 	pkt, err := parseReport(path, report)
 	if err != nil {
 		return err
 	}
 
+	pkt.Interfaces = append(pkt.Interfaces, &raven.User{ID: userID})
+
 	clientsMut.Lock()
 	defer clientsMut.Unlock()