Remove csscomb (soon replace it with something else) because scsslint has most of it covered already

v4: Drop .center-block for new-ish .m-x-auto 

Use text-emphasis mixin for .text-muted

- Both classes do about the same thing, but with different names
- Clarifies docs for .m-x-auto requiring a fixed width block element for it to work
- Add missing heading for clearfix section in docs (unrelated)

Strip out .err from our syntax highlighting styles because it fucks up our placeholders in code snippets

v4: Grid redux

v4: Native system font stack for sans-serif