Improve the way we observe changes on Users's collection

It is safer to look at subscriptions's state to users instead of depending
on the initial state of the collection at mounting time. Although
i couldn't force any race conditions on this, there's a chance it could
happen. The fix avoid this problem.
This complements #11533

bigbluebutton-html5/imports/startup/client/base.jsx

@@ -82,10 +82,8 @@ class Base extends Component {
 
     const {
       userID: localUserId,
-      credentials,
     } = Auth;
 
-    const { meetingId } = credentials;
     if (animations) HTML.classList.add('animationsEnabled');
     if (!animations) HTML.classList.add('animationsDisabled');
 
@@ -101,13 +99,12 @@ class Base extends Component {
       }, { fields: { name: 1, userId: 1 } }
     );
 
-    this.usersAlreadyInMeetingAtBeginning =
-      users && (typeof users.map === 'function') ?
-        users.map(user => user.userId)
-        : [];
-
     users.observe({
       added: (user) => {
+        const subscriptionsReady = Session.get('subscriptionsReady');
+
+        if (!subscriptionsReady) return;
+
         const {
           userJoinAudioAlerts,
           userJoinPushAlerts,
@@ -115,27 +112,25 @@ class Base extends Component {
 
         if (!userJoinAudioAlerts && !userJoinPushAlerts) return;
 
-        if (!this.usersAlreadyInMeetingAtBeginning.includes(user.userId)) {
-          if (userJoinAudioAlerts) {
-            AudioService.playAlertSound(`${Meteor.settings.public.app.cdn
-              + Meteor.settings.public.app.basename
-              + Meteor.settings.public.app.instanceId}`
-              + '/resources/sounds/userJoin.mp3');
-          }
+        if (userJoinAudioAlerts) {
+          AudioService.playAlertSound(`${Meteor.settings.public.app.cdn
+            + Meteor.settings.public.app.basename
+            + Meteor.settings.public.app.instanceId}`
+            + '/resources/sounds/userJoin.mp3');
+        }
 
-          if (userJoinPushAlerts) {
-            notify(
-              <FormattedMessage
-                id="app.notification.userJoinPushAlert"
-                description="Notification for a user joins the meeting"
-                values={{
-                  0: user.name,
-                }}
-              />,
-              'info',
-              'user',
-            );
-          }
+        if (userJoinPushAlerts) {
+          notify(
+            <FormattedMessage
+              id="app.notification.userJoinPushAlert"
+              description="Notification for a user joins the meeting"
+              values={{
+                0: user.name,
+              }}
+            />,
+            'info',
+            'user',
+          );
         }
       }
     });