Bug 829820 - Ability to mark selected SMS thread as Read/Unread

Never write the ``className`` directly, use the [classList](https://developer.mozilla.org/en-US/docs/Web/API/Element.classList) functions instead. That being said you should use  a custom data attribute instead (called ``action`` for example and with a value set to ``markasread`` and ``markasunread``) then use that to pick the icon in the CSS selector.