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

* Added @2x assets
* Added background-size declarations
* Added support for high resolution profile pictures (facebook+contacts)
* Some JS additions to handle drag effect (contact detail picture) in different devices
* Removed `responsive.js` as we are using a shared css file to handle multi-devices UI

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/8939
Please @jmcanterafonseca and @albertopq take a look.