Revert "Switch to using jekyll-assets to include assets"

This reverts commit 69c15fd4c42d51da3e69461790895cd9437965b9.

Conflicts:
	_config.yml
Move new stylesheet file away from jekyll-assets

This stylesheet was created after jekyll-assets was implemented, and now
that we're reverting that change this Sass file needs to be moved to the
correct place.
Merge pull request #275 from alphagov/revert-jekyll-assets

Revert jekyll-assets