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

* Updated @2x images
* Converted some px to rem rules
* Removed per device MQ

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/8980/files & https://github.com/mozilla-b2g/gaia/pull/8980/files please @lightsofapollo take a look!