Add configurable ratchicons directory.

This is the PR for issue [#711](https://github.com/twbs/ratchet/issues/711).

As it states there, we found our setup was trying to retrieve the ratchicons from `/assets/fonts/...` rather than at `/assets/ratchet/fonts/..` where they are actually located.

I've added a configurable sass variable so that the directory can be changed if need be.