Update bower.json to comply with new spec for `main`

Per https://github.com/bower/bower.json-spec/pull/43 :
> Use source files with module exports and imports over pre-built distribution files.

Also, the example in that PR doesn't include
/dist/movement.css or /dist/movement.min.css
in its `main`.

If you have a complaint about this, take it up with the Bower folks:
https://github.com/bower/bower.json-spec/issues

Per https://github.com/bower/bower.json-spec/pull/43 :
> font files [...] are not `main` files as they are not entry-points.

If you have a complaint about this, take it up with the Bower folks:
https://github.com/bower/bower.json-spec/issues

Grunt: extend `prep-release` task.

Make it so that everything is done for us to reduce the possibility of missing something when releasing.

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Closes #16151 by merging a rebased version of it that adds docs and 1 more assertion.

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Use CSS hack to fix IE8 .caret regression

Fixes #16124.

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Refs #15947

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Closes #16039.

affix plugin does not work within a pulled/pushed column

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Fix radio button toggle behavior for keyboard users

Fixes #16269.
Closes #16301 by merging a rebased version of it.

This documents the limitation discussed in https://github.com/twbs/bootstrap/issues/12126:

> When using column reordering and using the affix-plugin on an element positioned with push, the affix plugin doesn't work correctly - the element affected by the affix plugin is positioned to the left even when the element is in a column positioned on the right.

Seems to affect Safari (tested in version 8) only.

That issue has (unfortunately) been closed, so this at least documents the behavior.

Make .btn in .btn-toolbar float:left too

Add px unit to perspective property value

Reset text/font properties more thoroughly for tooltips+popovers

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as this breaks keyboard navigation for radio button toggles (see
https://github.com/twbs/bootstrap/issues/16223)

Use .dropdown-menu instead of ARIA roles for dropdown keydown event handler