FIX: ensures tag notification level is changed (#21106)

Following a change in e9f7262813 which prevents the notification level to be returned from the update endpoint, the model couldn't update itself. This commit makes the update manually and adds a test to prevent future regressions.

Note we could also change the backend endpoint, but this should work correctly with minimum risk.

app/assets/javascripts/discourse/app/controllers/tag-show.js

@@ -184,6 +184,8 @@ export default DiscoverySortableController.extend(
         .then((response) => {
           const payload = response.responseJson;
 
+          this.tagNotification.set("notification_level", notificationLevel);
+
           this.currentUser.setProperties({
             watched_tags: payload.watched_tags,
             watching_first_post_tags: payload.watching_first_post_tags,