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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

Reorganize utilities into more separate files under a `utilities` directory

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Remove useless .col-*-offset-12 and .col-xs-offset-0 classes

Apply clearfix to .card-{header,footer,block}
/fyi @mdo

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Docs: small tweaks to utilities

as on first skimming over these, it may not be completely obvious

...otherwise, by the same logic, it should be "Bolded text" above. just
looks clunky...

Fix bulleted lists on Utilities doc

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

Fixes #18365
Refs https://bugzilla.mozilla.org/show_bug.cgi?id=1228802

See also https://bugzilla.mozilla.org/show_bug.cgi?id=1228802

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Migration guide: .panel-title's closest replacement is .card-header

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Use variables for .progress base colors
/fyi @mdo

Refs #18535
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Refs #18535

Address IE10 in browser compatibility docs

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