Bug 830644 - Support WVGA and other non-HiDPI screen dimensions and resolutions

This PR is part of https://github.com/mozilla-b2g/gaia/pull/8380 because of the issues detected. Both should land together or at least Sms one should land first.

* sms: px to rem, added background size definitions, removed unnecessary css calcs
* responsive.css: adjusted mediaqueries to do not target desktop screens
* added @2x assets for sms

There is an issue that is more noticeable in Peak device about the send button that is misplaced about 15px down, do you know what can be wrong with this @borjasalguero ?

@rexboy7 @timdream @borjasalguero @jmcanterafonseca please review