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

 * Updated @2x images
 * Converted some px to rem rules

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 is the updated version of https://github.com/mozilla-b2g/gaia/pull/8737 fixing the issues addressed by @fcampo