* 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.

Fixes #24739

We might revisit this later, but currently we get too many FP with our current setup.

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

fixes #24744