Convert CommonJS exports to ES6-compatible exports

We use `export default` begrudgingly here. Ideally we'd use just `export`, though this entire SDK expects things to be exported as a default. Instead of breaking everything, we'll sacrifice our export pattern for a smaller diff - a later commit can always do the default export -> regular export conversion.

src/components/views/elements/DialogButtons.js

@@ -1,6 +1,7 @@
 /*
 Copyright 2017 Aidan Gauland
 Copyright 2018 New Vector Ltd.
+Copyright 2019 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -23,7 +24,7 @@ import { _t } from '../../../languageHandler';
 /**
  * Basic container for buttons in modal dialogs.
  */
-module.exports = createReactClass({
+export default createReactClass({
     displayName: "DialogButtons",
 
     propTypes: {