Require Signin

This PR adds url matching based on the link source url to allow iframes to run off different domains.

It also adds functionality that, if the class `.js-request-user-signin` is detected on the link element, will require the user to be signed in to view the iframe. It also takes care of displaying a signin link and revealing signin help text if provided.

The relevant areas in composer can be found via this PR: https://github.com/guardian/flexible-content/pull/727