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
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.
Use HTTPS in CDN URLs in _config.yml

For consistency with the rest of the docs and per our code guidelines.
Merge pull request #14156 from twbs/fix-14038

Fix popover dismiss-on-next-click example
Merge pull request #14192 from twbs/https-everywhere

Use HTTPS in CDN URLs in _config.yml
grunt dist
Document that form field feedback icons only work with `input.form-control`

Closes #14202.

[skip sauce]
Merge pull request #14203 from twbs/fix-14202

Document that form field feedback icons only work with input.form-control
update to rouge v1.5.1 in Travis CI build
update to Jekyll v2.1.1 in Travis CI build
Update grunt-csscomb.
Run `grunt`.
Merge pull request #13873 from twbs/grunt-csscomb

Update grunt-csscomb
Update Blob.js to the latest version.
Update Filesaver.js to v2014-07-21.
Update all devDependencies.