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.
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.
Document limitations of buttons JS plugin

Addresses #14148.
Use autocomplete="off" in buttons plugin docs examples

Refs #13875.
Have HTML5 validator ignore `autocomplete="off"` on input[type="checkbox"]

Not clear from the spec whether this is invalid per se.
But it is the current best known workaround for Firefox's unusual behavior.
Make warning from #13875 slightly more detailed

[skip sauce]
Use HTTPS in CDN URLs in _config.yml

For consistency with the rest of the docs and per our code guidelines.