fix hardcoded paths for fontawesome

Hi,
when spree is deployed under a sub-url (e.g. example.org/store) the urls to fontawesome do not work, because they are hardcoded.
This fix uses `font-url` instead of `url`, so that the urls to fontawesome are generated correctly, when spree is deployed under a sub-url.

There was a pull request with the same issue a few months back, but somehow the fix isn't in the current stable-2-0 (https://github.com/spree/spree/pull/2424).