[#16207] Uses the image-url function

- image-url() ensures the paths are correct, regardless of sub-dir
  layout of sass file
- also fixed font-urls
- still issues with h2 margin/padding
Merge commit 'f4a348a3cda342156e2e255e7aff9705f4ed7fa3' into 16207-fix-css-images
Merge pull request #153 from previousnext/16207-fix-css-images

[#16207] Uses the image-url function