fix(conference) fix not loading with older server versions

Use an iframe event which is fired really early to detect if the iframe
was loaded.

Incidentally the logic to detect if the iframe was actually loaded gets
simplified now, since the only way for the event to be received is for
the iframe to be loaded.

Fixes: #929

app/features/conference/components/Conference.js

@@ -230,7 +230,6 @@ class Conference extends Component<Props, State> {
 
         const options = {
             configOverwrite,
-            onload: this._onIframeLoad,
             parentNode: this._ref.current,
             roomName,
             sandbox: 'allow-scripts allow-same-origin allow-popups allow-forms'
@@ -241,6 +240,11 @@ class Conference extends Component<Props, State> {
             ...urlParameters
         });
 
+        // This event is fired really early, at the same time as 'ready', but has been
+        // around for longer.
+        // TODO: remove after a while. -saghul
+        this._api.on('browserSupport', this._onIframeLoad);
+
         this._api.on('suspendDetected', this._onVideoConferenceEnded);
         this._api.on('readyToClose', this._onVideoConferenceEnded);
         this._api.on('videoConferenceJoined',
@@ -323,25 +327,6 @@ class Conference extends Component<Props, State> {
             this._loadTimer = null;
         }
 
-        const frame = this._api.getIFrame();
-        const mainApp = frame.contentWindow.document.getElementById('react');
-
-        if (!mainApp) {
-            console.warn('Main application not loaded');
-
-            this._navigateToHome(
-
-                // $FlowFixMe
-                {
-                    error: 'Loading error',
-                    type: 'error'
-                },
-                this._conference.room,
-                this._conference.serverURL);
-
-            return;
-        }
-
         this.setState({
             isLoading: false
         });