Extends don’t work with libsass

Foundation 5 seems like it’s supposed to work with libsass, which is great! Definitely the right choice. Unfortunately, right now, `@extend` and `%` silent classes aren’t suppored in libsass. (The duplicate issues are https://github.com/hcatlin/libsass/issues/80, https://github.com/hcatlin/libsass/issues/137, and https://github.com/hcatlin/libsass/issues/146.)

The feature is supposed to be coming, but right now compiling Foundation with libsass will just output `@extend` and `%` literally. Since it’s being used so sparingly anyway, I think all the silent classes should be switched to mixins for the time being.