add <table> case to responsive-visibility mixin; see #10056
fix .btn-group-xs portion of #10979
fix invalid HTML in components.html

type="button" doesn't make sense for an <a>
switch to Sauce Labs for our cross-browser JS unit testing needs

Fixes #11087
Merge pull request #11701 from twbs/sauce-labs

switch to Sauce Labs for our cross-browser JS unit testing needs
fix broken link in CONTRIBUTING.md
Merge pull request #11697 from twbs/btn-xs-10979

fix .btn-group-xs portion of #10979
Merge branch 'master' into responsive-visibility-table

Conflicts:
	dist/css/bootstrap.min.css
Add a migration tip for .label to .label-default
Merge pull request #11705 from cyrusstoller/migration-guide

Add a migration tip for .label to .label-default
Dropping unused variable in dropdown.js

Dropping an unused variable in the Dropdown function inside the `dropdown.js` code.

There are no new/changed unit tests, as there are no feature/bug changes requiring new test cases.

For the record: I agree to dual-license this contribution under the Apache 2 and MIT licenses.
Add a migration tip for .label-important to .label-danger
Merge pull request #11709 from mdxs/patch-1

Dropping unused variable in dropdown.js