* 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 $enable-gradients is set to true
* use correct var
* fix focus shadows in button mixins
* Add opt-in gradients to alerts, buttons, carousel, custom radios and checkboxes, custom file input, and dropdown items
* Generate .bg-gradient- utilities
* add headings to colors page and document bg-gradient utils
* update the button color for active status, check with yiq as it's done for basic state and hover state
Use `display: block` since we're not really using any flex styles on the `.carousel-items` (only their controls and indicators). For the indicators, since moving to `width`, our indicators grow to fill available space, so we need to change `flex: 1 0 auto` to `flex: 0 1 auto` to prevent that.
- Drops the absolute positioning of the icons within the left/right controls. We have to keep the controls themselves positioned though since we're overlapping HTML elements here.
- No more position, left, right, or margins involved; just some justify-content and align-items.
- Add some comments for explaining which flex property-value pair does what.
- Remove the unapplied font and line-height stuff now that we're all SVGs and flexbox here.
This revamps the indicators to use flexbox instead of inline-block for added flexbility (hah). Indicators now automatically scale based on the number of elements present, and max out at the `$carousel-indicator-width` instead of always being that wide.
- fix naming of left/right controls
- drop the absolute positioning of things and rely on only 3d transforms
- remove img styles and require classes to avoid inline-block line-height stuff
* Remove comment that duplicated some code
* Use transition mixin whenever possible
* Create a new function to reduce duplication
* Use the new `breakpoint-infix` method
