Add aria-haspopup and aria-expanded attributes to dropdown plugin

1. Add aria-haspopup="true" and aria-expanded="false" to dropdown docs
2. Toggle aria-expanded between "true" and "false" when opening and closing dropdown

Satisfy item 1 from https://github.com/paypal/bootstrap-accessibility-plugin#dropdown. See #13553.
add docs note about browsers w/ JS disabled; fixes #14134

[skip sauce]
Another new js bin link

/cc @cvrebert @hnrch02
Merge pull request #14143 from twbs/fix-14134

add docs note about browsers with JS disabled
Fix jsbin link
Merge pull request #14151 from BBosman/fix-link

Fix jsbin link
popover dismiss-on-next-click example: instead of <button>, use <a> w/ tabindex

Fixes #14038.

Research supporting this choice of approach:
    https://gist.github.com/cvrebert/68659d0333a578d75372

Why not <button>?
Even with a `tabindex` attribute, they don't gain focus when clicked upon in:
    * Mac OS X Firefox 30.0
    * Mac OS X Safari 7.0.5
regenerate docs/assets/js/docs.min.js
Follow-up to #13907: simplify JS logic for focus shim
Clean up Affix documentation
Clean up the Alert documentation
Merge alert examples and add `.bs-example-standalone` class
Merge pull request #14160 from hnrch02/simplify-button-focus-logic

Follow-up to #13907: simplify JS logic for focus shim
speed up js tests a bit
Tabs to spaces
Merge pull request #14171 from wangsai/tab2space

Tabs to spaces in variables.less
Merge pull request #14055 from twbs/fat-speed-up-tests

speed up js tests a bit
Add a bottom affixed element to affix visual tests.
Refactor determining affix state into a separate expanded method
  in order to handle multiple edge cases, specifically when the document
  height is dynamic.
Always reposition an affix that is affixed to the bottom.
Fix issue where bottom affixed element floats over the footer when the
  document height is smaller than the viewport height.