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/context_menus/MessageContextMenu.js

@@ -37,7 +37,7 @@ function canCancel(eventStatus) {
     return eventStatus === EventStatus.QUEUED || eventStatus === EventStatus.NOT_SENT;
 }
 
-module.exports = createReactClass({
+export default createReactClass({
     displayName: 'MessageContextMenu',
 
     propTypes: {