This commit allows nested structures for accordions. Also a part of
the documentation about data-children is removed because this
functionality didn't work and it's not applicable anymore.

Tests with the collapse accordion are also a bit adjusted to the new
situation.

"Choose file ..." text is now written inside the label tag, so the translation is outside the css scope. 
$custom-file-text variable should only contain "Browse" translations.

Closes #25067

- Changes the wrapping label to a div so we can style the label instead of another element while also supporting form validation.
- Fixes form validation styles for custom file input (closes #24831).
- Updates docs with validation styles (also adding example feedback text while I was there) and new how it works section.

Adds some additional context on `.form-group` usage

* Restyle code element, clean up variables

- Removes the padding and background-color
- Changes the color to a brand color ()
- Add new variables to kbd element styles to account for removing the code ones
- Remove overrides that were needed previously

* only break between words, and override it in the pre

* make file inputs 100% wide

* scope custom file changes in input group for sizing

* shorter button labels for responsive

* narrower cards

* button was too wide

* downsize

* fewer links in pagination

* redo cell text

* improve overflow docs

* match layout behaviors

* ditch the indicator as separate element for psuedo-elements on the label

* move disabled to attribute only on input

* redo default inline check to support new markup

* redo inline forms

* clean up vars

* update validation mixin to new structure

* update checks in docs

* linting for for/id attributes

* Rewrite input group component

* Set the feedback to 100% width for input group

* Move from .row to .form-row for tighter layout

* no need for custom feedback here, we're using browser messaging

* add input group to validation examples

* add note about validating multiple

* migration note added

* 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...

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).

* 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

Fixes #24739