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.
Merge branch 'dropdown-aria' of git://github.com/mckramer/bootstrap into mckramer-dropdown-aria

Conflicts:
	docs/_includes/js/dropdowns.html
Merge pull request #14208 from twbs/mckramer-dropdown-aria

Mckramer dropdown aria