as role="menu" is a very specific (and strict) ARIA pattern for
desktop-like application menus, and our dropdowns are often used
as pure navigation dropdowns, this change abandons ARIA menus for
a more open-ended and light-weight approach
(see http://heydonworks.com/practical_aria_examples/#submenus and
http://www.w3.org/WAI/tutorials/menus/flyout/#improve-screen-reader-support-using-wai-aria)
note that in dropdown.js, switched to now target ``.dropdown-menu``
instead of ``role["menu"]`` - this also prevents bootstrap scripts
from "bleeding" into non-bootstrap components on the same page.
also removed the ``role=["listbox"]`` part, which appears to be
vestigial/unused (only place in bootstrap that uses that
role are carousels, and their key handling is done separately)

Closes #14726.

faster and easier to test/develop js functionality not represented in unit tests, and gives us
a playground for interactions, etc.

It also makes it so developing javascript is now decoupled form jekyll, which should make everything
faster and less painful.

This commit also reverts my filter commit https://github.com/twbs/bootstrap/commit/9900771aa7f1b3ddcee49aec84082104776ace70
which broke scrollspy for dropdowns.