Closes #24437, closes #22457

* 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

Merge branch 'btn-active' of https://github.com/andresgalante/bootstrap into andresgalante-btn-active

IE10 and IE11 do not properly support the use of margin-auto on flex parents with a justify-content value other than flex-start. This PR removes the examples showcasing those two classes at play together in favor of simpler ones with _just_ the margin-auto utilities.

More details can be found at https://stackoverflow.com/a/37535548, which has been linked to from the docs as well.

Closes #24319 and closes #24413.

This document contains the word JavaScript everywhere apart from in one place where it's abbreviated to JS. This just makes sure it's written as JavaScript consistently across the page.

This way, descenders are not cropped when truncating. Fixes #23956.

double use of see

* font-sizes relative to font-size-base

* ditch $heading-font-size-base, simplify

* apply same to .lead var

* also update progress bar font-size var

* Refer to utility classes uses. Add Printing section

refer to (not navbar) classes used for newbies
navbars don't print, so add example of a print-only alternative

* typos, add links

typo in {% example %}
add links to component pages

* grammar

* Add a dropdown with divider to the main example

* change id name to stop travis error

I assume id="camelCase" is the standard for id names, rather than the clearly superior id="underscore_words"

* review changes

still like to emphasise the foreign utility classes in the main example
bg-light's name is self explanatory (so its obvious where to look it up), but mr-sm-2's name isn't at first

* review changes

* grammar

* .fixed-top/bottom don't reserve space on the screen

... as they are position:fixed. 
better wording welcome

* copyediting

- rewrite utils line and link to the utils pages
- restore the paragraphs to the plac...