Bug 943825 - [Gaia] Mount/Unmount SD card

User story for SD card management.
To make the selectors be maintained easier, they better reflect the DOM structure and be consistent. In this case, note that `header` and `rootElement` are siblings but have different type of selectors. One is using dataset and one is using id. I would suggest to add dataset to both `header` and `rootElement`, then we can use `header[data-id="..."]` and `ul[data-id="..."]` as selectors.