Add rule to enforce spacing in curly brackets in JSX children

This required an updated in version of eslint-plugin-react to 7.4.0.

src/components/views/rooms/SimpleRoomHeader.js

@@ -26,7 +26,7 @@ export function CancelButton(props) {
     return (
         <AccessibleButton className='mx_RoomHeader_cancelButton' onClick={onClick}>
             <img src="img/cancel.svg" className='mx_filterFlipColor'
-                width="18" height="18" alt={_t("Cancel")}/>
+                width="18" height="18" alt={_t("Cancel")} />
         </AccessibleButton>
     );
 }