Show reaction title and shortcode on hover

This shows the title and shortcode for the hovered reaction at the bottom of the
tooltip. If nothing is hovered, a blank space is shown for now, but will
eventually become a link to a full emoji picker in future work.

Part of https://github.com/vector-im/riot-web/issues/9753

src/components/views/messages/ReactionTooltipButton.js

@@ -57,6 +57,7 @@ export default class ReactionTooltipButton extends React.PureComponent {
         });
 
         return <span className={classes}
+            data-key={content}
             title={this.props.title}
             aria-hidden={true}
             onClick={this.onClick}