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.
Clean up Affix documentation
Clean up the Alert documentation
Merge alert examples and add `.bs-example-standalone` class
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.
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`.