v4: Switch to jQuery Slim

[mdo]

This supersedes #20203, changing our jQuery version to the latest slim version while updating our examples to match the new CDN and what not.

[Starsam80]

I think docs/assets/js/vendor/jquery.min.js and js/tests/vendor/jquery.min.js need to be updated as well

[mdo]

Hmm, wonder if we need to keep those js/tests/vendor/ copies around? I feel like we should be able to reference the docs copies so we don't have duplicate files.

[bardiharborow]

js/tests/vendor/jquery.min.js is only referenced in js/tests/index.html, so I suppose you could use ../../docs/assets/js/vendor/jquery.min.js. If you do this, you should do the same to Tether.

[bardiharborow]

You're missing js/tests/index.html#L9-L10, and then LGTM.