* 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
* 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
* fixes migration forms guides
* removes container from Horizontal form
* adds col-form-label sizes on form docs
* removes .form-control-label from forms.scss
* add note to migration
* use a trailing slash when possible
* use https when possible
* remove a few redirected links
* consistently use `https://popper.js.org/`
* fix `iconUrl` in nuget files
* change Jekyll Windows guide to the official one
