Updates openMessageDialog to get only key and options.

conference.js

     }
 
     if (showThankYou) {
-        APP.UI.messageHandler.openMessageDialog(null, null,
-            APP.translation.generateTranslationHTML(
-                "dialog.thankYou", {appName:interfaceConfig.APP_NAME}));
+        APP.UI.messageHandler.openMessageDialog(
+            null, "dialog.thankYou", {appName:interfaceConfig.APP_NAME});
     }
 
     if (!config.enableWelcomePage) {

modules/UI/UI.js

  * @returns {Promise} Resolved with value - false if the dialog is enabled and
  * resolved with true if the dialog is disabled or the feedback was already
  * submitted. Rejected if another dialog is already displayed. This values are
- * used to display or not display the thank you dialog from 
+ * used to display or not display the thank you dialog from
  * conference.maybeRedirectToWelcomePage method.
  */
 UI.requestFeedbackOnHangup = function () {
 UI.showExtensionRequiredDialog = function (url) {
     messageHandler.openMessageDialog(
         "dialog.extensionRequired",
-        null,
-        APP.translation.generateTranslationHTML(
-            "dialog.firefoxExtensionPrompt", {url: url}));
+        "dialog.firefoxExtensionPrompt",
+        {url: url});
 };
 
 /**
     messageHandler.openTwoButtonDialog({
         titleKey: 'dialog.externalInstallationTitle',
         msgKey: 'dialog.externalInstallationMsg',
-        msgString: '',
         leftButtonKey: 'dialog.goToStore',
         submitFunction,
         loadedFunction: $.noop,
     messageHandler.openMessageDialog(
         "dialog.error",
         stream.isAudioTrack()? "dialog.micNotSendingData" :
-            "dialog.cameraNotSendingData",
-        null);
+            "dialog.cameraNotSendingData");
 };
 
 UI.updateDevicesAvailability = function (id, devices) {

modules/UI/util/MessageHandler.js

      *
      * @param titleKey the key used to find the translation of the title of the
      * message, if a message title is not provided.
-     * @param messageKey the key used to find the translation of the message,
-     * if a message is not provided.
-     * @param message the message to show. If a falsy value is provided,
-     * messageKey will be used to get a message via the translation API.
+     * @param messageKey the key used to find the translation of the message
+     * @param i18nOptions the i18n options (optional)
      * @param closeFunction function to be called after
      * the prompt is closed (optional)
      * @return the prompt that was created, or null
      */
-    openMessageDialog: function(titleKey, messageKey, message, closeFunction) {
+    openMessageDialog:
+        function(titleKey, messageKey, i18nOptions, closeFunction) {
         if (!popupEnabled)
             return null;
 
-        if (!message) {
-            message = APP.translation.generateTranslationHTML(messageKey);
-        }
-
-        let dialog = $.prompt(message, {
+        let dialog = $.prompt(
+            APP.translation.generateTranslationHTML(messageKey, i18nOptions), {
             title: this._getFormattedTitleString(titleKey),
             persistent: false,
             promptspeed: 0,
                     closeFunction(e, v, m, f);
             }
         });
-        APP.translation.translateElement(dialog);
+        APP.translation.translateElement(dialog, i18nOptions);
         return dialog;
     },
     /**