Remove the fade in image in favour of background gradient

* Introduces vertical-gradient mixin (tried and tested
in MobileFrontend)
* Add additional LESS variables
* Just dont give versions with no vertical gradient support a fade
effect - it's not a big deal
* Save a single asset download

Change-Id: Ie36708f32edf0d83a0be843c115ad82d8014e4e1
Merge "Remove the fade in image in favour of background gradient"