Bug 998847 - [Settings] refactor storage.js with AMD pattern

- move dialog into root panel
- rename id to class
- fix marionette test
- un-registered observer when leaving the panel
- add enabled to observer/unobserve
- create separated modules for each menu item
- move usb and media item together
- fix binding
We should use the bound version of `this._umsSettingHandler` as the function uses `this`.