Bump the frontend toolkit from 0.12.6 to 0.46.0

This might look like a big jump in version numbers but the diff
isn't that huge.

https://github.com/alphagov/govuk_frontend_toolkit/compare/ef84748...44d129d
Set $path correctly in Sass

This points to the base directory where Sass should look for images.
Remove separator.png from assets directory

This is already copied over into the public directory from the
govuk_template, so it's just duplicated here for no good reason.
Use file-path mixin for our own assets

This doesn't give us cachebusting, but it's a nice enough solution
for the time being.
Merge pull request #313 from alphagov/compile-sass-image-paths-correctly

Compile Sass image paths correctly