Remove box-sizing polyfill from mixins.

If needed, this needs to be applied explicitly in main/_oldie.scss
using the %box-sizing-polyfill extend. Also, the polyfill is no
longer copied to the deployment dir per default.
Move IE7 helper classes fallbacks to main/_oldie.scss.

Remember: This means if IE7 needs to be supported, we need to use
classes instead of extends.
Drop IE7 styles for button mixins.
Replace IE7 hack with using conditional class.
Drop inblock-level mixin, use extend instead.
Exclude oldie and fontello styles from scss-lint.

_oldie.scss will never be lint-free because of behaviour. fontello
styles are generated remotely, it would be too much of a hassle to
make them lint-free (like any 3rd-party scss).