v4: Update modal sizing

- Switch from width to max-width for all widths to avoid scaling outside viewport bounds
- Rejigger the media queries for a more logical breakpoint for the large and small modal sizes
- Avoids changing the width of the default modal (nullifying #17794 and fixing #17581)

v4: Fix row-based <dl>

- Using the grid classes, they still need clearfixes since rows aren't injected between them all
- Update docs to fix broken classes

v4: Revamp list group items built with anchors and buttons

- Make it an explicit class instead of qualifying with elements (.list-group-item-action)
- Rearrange the entire file for more straightforward flow
- Fix text-decoration bug as a result of the reorder
- Switch from h4's to h5's in docs
- Update docs to merge anchors and buttons sections; clearify usage guidelines there, too
- Nullify #17479 in the process

v4 - The .text-hide won't hide the text :)

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