update classes to follow talk naming convention

plugins/talk-plugin-permalink/client/components/PermalinkButton.js

@@ -73,7 +73,7 @@ export default class PermalinkButton extends React.Component {
             ref={(ref) => this.linkButton = ref}
             onClick={this.toggle}
             className={cn(`${name}-button`, styles.button)}>
-            <span className='permalinkButtonLabel'>
+            <span className='talk-plugin-permalink-button-label'>
               {t('permalink')}
             </span>
             <Icon name="link" className={styles.icon}/>