* 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
1. Explicitly disable ESLint in Hound for now; it was disabled all this time. Temporarily disabled until the ESLint plugins are updated upstream
2. Remove unneeded ignore from .houndignore
* use descriptive docs btn naming
* rename `btn-bd-yellow` to `btn-bd-download`
* sort state vars
* sort state vars
* info, warning, danger
* rename btn-bd-purple
* rename `.btn-bd-purple` to `.btn-bd-primary`
* align docs color variables
* generate $bd-purple-light color
* with http://razorltd.github.io/sasscolourfunctioncalculator/
* allow overide of docs variables
