Add tabIndex to the button to make it keyboard focusable

Signed-off-by: Marco Zehe <marcozehe@mailbox.org>

src/components/views/messages/ReactionsRowButton.js

@@ -129,6 +129,7 @@ export default class ReactionsRowButton extends React.PureComponent {
         return <span className={classes}
             role="button"
             aria-label={label}
+            tabindex="0"
             onClick={this.onClick}
             onMouseOver={this.onMouseOver}
             onMouseOut={this.onMouseOut}