* chore: sync with twbs (#1)

* Make styling of grouped option buttons class-based

Referencing #23728, styling grouped radio and checkbox buttons should not require the data-toggle attribute.

* Fix installing deps in our webpack docs

* Fix nested cards on card group (#24766)

* Update popper.js to v1.12.9. (#24797)

* Don't run postcss for `dist/css/*.min.css` files. (#24676)

* Change the text color to dark on bg color warning examples (#24805)

* Remove unused file and bundle ie-emulation-modes-warning.js with docs.min.js (#24825)

* Remove the unused assets/js/ie10-viewport-bug-workaround.js.

* Combine ie-emulation-modes-warning.js with docs.min.js.

* Typos in grid.md (#24828)

Fixed typos in offsetting section of grid.md

* Fix dropup example code

* Update devDependencies. (#24829)

* Add support for fractional viewport widths (zoom/high-dpi displays) (#24299)

* Change breakpoint  max- calculation to fr...

The issue has been fixed.

Note that we should revert this when the issue is fixed.

We could tell folks to nix the class, we could tell them to override it, or we could add another modifier of some kind to address this. None of them seem particularly useful, but the padding override feels the most approachable and clearly documentable. Added this here to close #24844.

* Fix docs horizontal scrollbar in Components > Modal

* Add overflow to docs example class

* Drop the handleUpdate() variant

While I understand the variant covers the "get a particular plugin instance" case from http://getbootstrap.com/docs/4.0/getting-started/javascript/#programmatic-api it seems weird that we do it here but not everywhere else in the docs (randomly checking other pages, we seem to stick to the documented methods as written on those pages). As this isn't properly explained as is, and as it adds nothing (more of a preference for authors), it would be easier to just drop the variant and stick with the documented method as outlined later on in the same page (avoids having to go off on a "why should you use this instead of that" tangent here).

Also, include it in the `css` npm script since it's instant.

Merge branch 'btn-group-styling' of https://github.com/picapixa/bootstrap into picapixa-btn-group-styling

* Fix Issue #24144

Alter button-outline-variant mixin to darken hover and active background in same fashion as filled button.

* Fix Issue #24144

Filled button on hover should utilize the same color-yiq mixin so that theme buttons match

* default values, fix mixin

1. Use the same jQuery version in jspm
2. Update the files to only include the source and dist folders

* Add support for size attribute on custom selects

* Add large custom select, document it and the small variant

* fix custom select focus state

* fix custom file input focus styles

* remove empty line

* Rewrite custom form check backgrounds

Previously, this was all just a background-color and background-image. When we restored the gradients though, we had two background-images competing on the same element, causing rendering glitches. This refactors that code, creating a mixin to simplify things, so we can we easily use two background-images (SVG icon and gradient) when -gradients is set to true.

Fixes #24598

* restore default vars

* Revamp custom check and radio backgrounds

Instead of applying multiple background-image's to the same element, we're adding a new ::before pseudo-element to layer the background-images. Gradients go on the .custom-control-indicator while their icons go on the ::before element. This allows us to shave some bytes from when we compile and we previously needed to redeclare the background-image for the icon if you changed the gradient.

* remove now unused mixin

* mention change in migration docs

* Change breakpoint  max- calculation to fractional values
* Update docs to reflect fractional max-width breakpoint values
* Add fractional max-width to offcanvas example
* Add documentation and SCSS comment for fractional viewport support

Fixed typos in offsetting section of grid.md

* Remove the unused assets/js/ie10-viewport-bug-workaround.js.

* Combine ie-emulation-modes-warning.js with docs.min.js.