add role="button" to a[data-toggle="collapse"]

Add aria-expanded to navbar toggles

Also, use id rather class name for the navigation, as it's unique and
lets us use correct aria-controls (and without the use of id,
aria-expanded does not get updated correctly by the current dropdown
toggle script)

Callout for tooltips and keyboard/AT accessibility

Before, most callouts hierarchically came under Examples > Four
directions, which is not appropriate. Now they're under Markup

Include callout explicitly mentioning tooltips should only be added to
focusable elements. Closes #16134

Closes #16073 by merging it

Increment quick start options to six in readme.md

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Closes #16113
Closes #16114

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Add `.alert-warning` related change to migration doc

avoid unnecessary HTML string building+parsing

Update jspm configuration

Refs #15947

ScrollSpy: Fixed proper nav element selection when scrolling upwards

bootstrap-sass v3.3.2 is broken.
Refs https://github.com/twbs/bootstrap-sass/issues/862
Refs https://github.com/twbs/bootstrap-sass/releases/tag/v3.3.3
Refs https://github.com/twbs/bootstrap-sass/commit/daeb43dcc7b0ab06328acaca0549ee68c039aaa6

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Increment quick start options in readme.md to five

Add Wall of Browser Bugs entry for #16022

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