Make $wgMFDeviceWidthTablet and $wgMFDeviceWidthMobileSmall configurable

* Order variables properly
* Make it possible to override LESS variables
* Unset $cwd ASAP
* Add documentation

Change-Id: Ifde743a5cbd203360f424fc19b9717e9e99224db
Merge "Make $wgMFDeviceWidthTablet and $wgMFDeviceWidthMobileSmall configurable"