add divider between tabs and regular buttons in room header

src/components/views/right_panel/HeaderButtons.js

@@ -88,7 +88,7 @@ export default class HeaderButtons extends React.Component {
 
     render() {
         // inline style as this will be swapped around in future commits
-        return <div style={{display: 'flex'}}>
+        return <div className="mx_HeaderButtons">
             { this.renderButtons() }
         </div>;
     }