The outline button variant mixin should use the hover mixin, as does the non-outline version.

When gradients are enabled there is still a hover state on disabled
buttons since the hover rules apply to background-image and disabled
rules apply to background-color. This applies the logic already present
in dropdowns to buttons. This fix was originally proposed by @ysds.

Closes https://github.com/twbs/bootstrap/issues/25316

* Base the outline button :active color on the background, fixing a contrast issue

* Only apply focus outline to active when it's focused

* 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

Fixes #24503 by manually calling the border-radius instead of using the mixin.

* Update the form focus mixin to use a manual `$enable-shadows` check so we can always ensure a focus state for accessibility and consistency

* - Add new `$input-btn-focus-width` and `$input-btn-focus-color` variables.

- Replace separate `$btn-focus-box-shadow` and `$input-focus-box-shadow`
variables with unified `$input-btn-focus-box-shadow` to match our
combined variables approach elsewhere.

* Put new focus width var to use in buttons mixins

* use new button input-box shadow var

* Add a new mixin for quickly adding linear gradient to components when $enable-gradients is set to true

* use correct var

* fix focus shadows in button mixins

* Add opt-in gradients to alerts, buttons, carousel, custom radios and checkboxes, custom file input, and dropdown items

* Generate .bg-gradient- utilities

* add headings to colors page and document bg-gradient utils

* update the button color for active status, check with yiq as it's done for basic state and hover state

function only returns a value, not the attribute itself
updated every use of the former mixin to use the new function

* use a mixin there
* revamp .btn-link to match default and outline variants
* remove unnecessary properties

remove - vars, update docs to generate from data yml files, update mixins, and generate card, button, bg, and color classes from the sass map

* Fixed some linting issues

* Run npm tasks after scss cleanup

* Revert "Run npm tasks after scss cleanup"

This reverts commit 1103a0da.

* Property sort order for grid

* Let's respest the property order in the mixins

* Respect property sort order in reboot file

* ::-ms-expand is a vendor-prefix, add it to the scss-lint disable

* Revert hover mixin comment

* Fixed missing mixin hover-focus

don't implement for the outline style because outline buttons get no shadow by default

- removes original outline removal (hah)
- replaces it with an explicit `outline: 0` on `.btn`
- instead of replicating `:hover` for `:focus`, uses custom and themed `box-shadow` for an "outline"
- not mapped to the `$enable-shadows` variable because accessibility

Trying to simplify our output here by revamping these selectors. We overcomplicated things by setting hover styles for nearly every state (disabled and active included), and we set them in the wrong order.

This commit does the following:

- Reorders states so disabled comes before active, thereby removing the need to set disabled-active styles.

- Removes all focus and hover styles from disabled states as those will naturally inherit from the default button state.

- Renamed `.open` to `.show` to fix dropdown toggle highlighting.

- Tweaked some indendation in the Sass.

Fixes #20609

PR #20641, another solution to this issue, hard-codes use of $body-color, which may not be the desired color for the text when hovering.

- Rename -height to -height-base to match other vars
- Drop use of -height across the board and rely on it to be inherited
- Adjust padding of .dropdown-header to account for different line-height of h6 heading element (this needs refactoring for variables and rems also)

[skip sauce]
[skip validator]

If we want to support namespaced import:

```scss
.twbs {
  @import 'bootstrap';
}
```

We cannot use `selector &`, see sass/sass#1817.

`fieldset[disabled] &` is not needed as we no longer support IE8.