Copy assets into theme

Currently, the theme CSS references Gov UK images within the node_modules directory, however if you're not compiling the CSS yourself and installing the dependencies, they won't work.

The Gulp script should be modified to include a task which extracts the images from within node_modules and moves them into an assets folder within the theme, and they should be committed into the repository by default.

The new assets folder should then be referenced from within the templates where necessary, though the directory twig variable should be changed to always reference the GovUK theme itself (and not a child if extended).

I've added a patch which implements this, though the assets need to be regenerated and committed (CSS and images/fonts).