fix(prejoin):Disable device selection on iosSafari

It seems that showing the device selection dialog on ios Safari will
leads to not working audio. This is temporary fix until we find out
better solution.

react/features/device-selection/functions.js

     // conference and this is not supported, when we open device selection on
     // welcome page changing input devices will not be a problem
     // on welcome page we also show only what we have saved as user selected devices
-    if (!conference) {
+    if (!conference && !isMobileSafari) {
         disableAudioInputChange = false;
         disableVideoInputSelect = false;
         selectedAudioInputId = userSelectedMic;