Drop the event source if we are unauthorized (#15275)

A previous commit that sent unauthorized if the user is unauthorized
simply leads to the repeated reopening of the eventsource. #

This PR changes the event returned to tell the client to close the
eventsource and thus prevents the repeated reopening.

Signed-off-by: Andrew Thornton <art27@cantab.net>

web_src/js/features/stopwatch.js

@@ -55,6 +55,11 @@ export async function initStopwatch() {
         });
         worker.port.close();
         window.location.href = AppSubUrl;
+      } else if (event.data.type === 'close') {
+        worker.port.postMessage({
+          type: 'close',
+        });
+        worker.port.close();
       }
     });
     worker.port.addEventListener('error', (e) => {