Drop the event source if we are unauthorized (#15275) (#15280)

Backport #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/eventsource.sharedworker.js

@@ -10,6 +10,7 @@ class Source {
     this.listening = {};
     this.clients = [];
     this.listen('open');
+    this.listen('close');
     this.listen('logout');
     this.listen('notification-count');
     this.listen('stopwatches');