Compile Sass image paths correctly

This is alternative to #306 which uses less Ruby and the thing @edds made in https://github.com/alphagov/govuk_frontend_toolkit/commit/096ec3607fb160f82cd99e6d9ea7e338051df2b5.

It needs #310 to be merged first and can then be rebased against master.

I think it's pretty cool and has made me a lot happier about our asset compilation.