Docs: add accessibility callout for tooltips

Fix typo in docs/getting-started/introduction.md

Remove 'including'.

Docs: change "Button" to "Go somewhere"

"Go somewhere" is more descriptive of an example link, rather than a
button that triggers in-page functionality. Also makes it consistent
with all other examples in this page

Docs: add aria-disabled to disabled link example and description

Docs: add aria-pressed=true to components buttons active state example

Docs: add ellipsis to modal example code

Fix syntax of .text-hide's `font` property value

...as per the text immediately preceding this example

In SCSS, the quotes were included verbatim in the resulting CSS, which isn't valid syntax for the `font` property.
Removing the quotes fixes the syntax error and does not cause any SCSS compiler error.

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Un-nest Autoprefixer settings in module object

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See https://www.mozilla.org/en-US/firefox/organizations/faq/

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Refs #18584
Closes #18659

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Exempt getUID() from ESLint's no-bitwise rule

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Add Tether to example pages that were missing it

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Include Tether via CDN in starter templates

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dropdown.js: Use more straightforward phrasing for index bound check

Modal: Ignore spurious focus event that Firefox fires at document when switching back to its tab

Docs: reintroduce `aria-describedby` advice for help text

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Based on v3 http://getbootstrap.com/css/#forms-help-text, adapted to
make examples more "real life"

Introduce $btn-block-spacing-y variable

Uncomment Firefox <progress> styles
/fyi @mdo

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Refactor Breadcrumbs component

Use new `.breadcrumb-item` class instead of child selectors and `li` tag selectors,
so as to no longer require the usage of `<ol>`-based markup.

Rename variables to follow naming conventions:
* $breadcrumb-padding-vertical => $breadcrumb-padding-y
* $breadcrumb-padding-horizontal => $breadcrumb-padding-x

Introduce new variable: $breadcrumb-item-padding

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Update shrinkwrap