Bug 943825 - [Gaia] Mount/Unmount SD card

User story for SD card management.
A class name, for example, 'format-btn`, is sufficient for the button to be selected rather than an id as the button is already under the root element of a volume.

By using a class name we can decouple the selector from l10n-id that is tend to be changed frequently, and then we can select the button by `this.rootElement.querySelector('.format-btn')`.