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

**Costcontrol** updates:
* Updated @2x images
* Converted some px to rem rules
* Added progres_activity.css from shared to index.html

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/8883 fixing the issues addressed by @lodr