Fixes issue with conference starting before the UI is ready + enableWelcomePage config support.

app.js

 };
 
 function init() {
-    APP.UI.start();
-    APP.conference.init({roomName: buildRoomName()}).then(function () {
-        APP.UI.initConference();
+    var isUIReady = APP.UI.start();
+    if (isUIReady) {
+        APP.conference.init({roomName: buildRoomName()}).then(function () {
+            APP.UI.initConference();
+
+            APP.UI.addListener(UIEvents.LANG_CHANGED, function (language) {
+                APP.translation.setLanguage(language);
+                APP.settings.setLanguage(language);
+            });
 
-        APP.UI.addListener(UIEvents.LANG_CHANGED, function (language) {
-            APP.translation.setLanguage(language);
-            APP.settings.setLanguage(language);
+            APP.desktopsharing.init(JitsiMeetJS.isDesktopSharingEnabled());
+            APP.statistics.start();
+            APP.connectionquality.init();
+            APP.keyboardshortcut.init();
+        }).catch(function (err) {
+            console.error(err);
         });
-
-        APP.desktopsharing.init(JitsiMeetJS.isDesktopSharingEnabled());
-        APP.statistics.start();
-        APP.connectionquality.init();
-        APP.keyboardshortcut.init();
-    }).catch(function (err) {
-        console.error(err);
-    });
+    }
 }
 
 /**

modules/UI/UI.js

     $(window).resize(onResize);
 }
 
+/**
+ * Starts the UI module and initializes all related components.
+ *
+ * @returns {boolean} true if the UI is ready and the conference should be
+ * esablished, false - otherwise (for example in the case of welcome page)
+ */
 UI.start = function () {
     document.title = interfaceConfig.APP_NAME;
     var setupWelcomePage = null;
             setupWelcomePage = require("./welcome_page/WelcomePage");
         setupWelcomePage();
 
-        return;
+        // Return false to indicate that the UI hasn't been fully started and
+        // conference ready. We're still waiting for input from the user.
+        return false;
     }
 
     $("#welcome_page").hide();
         SettingsMenu.init(eventEmitter);
     }
 
+    // Return true to indicate that the UI has been fully started and
+    // conference ready.
+    return true;
 };