Add integrity check to assets files served by CDN

We can add the [`integrity` attribute to the ``](https://developer.mozilla.org/en-US/docs/Web/Security/Subresource_Integrity). Sprockets already [supports that feature](https://www.rubydoc.info/github/rails/sprockets/Sprockets/DigestUtils:integrity_uri), the only required change is on the `stylesheet_tag` helper.

_Originally posted by @eloyesp in https://github.com/freeCodeCamp/devdocs/issues/1112#issuecomment-543292784_