2220 subtheming for agov zen

This pull request makes using agov_zen easier as both a base theme and for re-theming.

* Any non-grayscale colours, and fonts, have been removed from agov_zen
* The standard Barton colour scheme has been re-added via a subtheme
* A subtheme starterkit has been added. This supports SASS.
* The way colours are specified in agov_zen has changed from using variables with colour names ("$blue", for example) to descriptive names (e.g. "$color-primary").

Fixes included in the base theme:
* Padding around several page elements has been normalised across breakpoints (e.g. adding extra padding to the header when on small screens, to stop the logo from touching the side)
* The text-resize widget had some bugs which have been fixed
* Updated screenshots