Changes display name prompt to non modal, so people can mute/unmute.

react/features/display-name/components/web/DisplayNamePrompt.js

@@ -56,7 +56,7 @@ class DisplayNamePrompt extends AbstractDisplayNamePrompt<State> {
     render() {
         return (
             <Dialog
-                isModal = { true }
+                isModal = { false }
                 onSubmit = { this._onSubmit }
                 titleKey = 'dialog.displayNameRequired'
                 width = 'small'>