feat(ShowMore): Add classname to show more/show less button (#6904)

app/javascript/mastodon/components/status_content.js

@@ -158,7 +158,7 @@ export default class StatusContent extends React.PureComponent {
           <p style={{ marginBottom: hidden && status.get('mentions').isEmpty() ? '0px' : null }}>
             <span dangerouslySetInnerHTML={spoilerContent} />
             {' '}
-            <button tabIndex='0' className='status__content__spoiler-link' onClick={this.handleSpoilerClick}>{toggleText}</button>
+            <button tabIndex='0' className={`status__content__spoiler-link ${hidden ? 'status__content__spoiler-link--show-more' : 'status__content__spoiler-link--show-less'}`} onClick={this.handleSpoilerClick}>{toggleText}</button>
           </p>
 
           {mentionsPlaceholder}