Bug 830644 - [Email][Hidpi]Support high resolution devices

* Added @2x assets
* Added background-size declarations

In order to test this PR you need to:
 * **HIDPI Device:** `HIDPI=1 make install-gaia` 
 * **non-HIDPI Device:** `make install-gaia` 
 * **B2G desktop:** run it with `--screen=480x800` flag

This PR has been created from this source: https://github.com/mozilla-b2g/gaia/pull/8867 Please @dominickuo  and @asutherland  take a look.