Support absolute URL as data-interactive attribute

This is part of my work on getting interactives into Composer using the data-interactive/boot.js design we've been discussing (for about a decade) with @commuterjoy. This PR also whitelists element-interactives, as per the recent PR by @rich-nguyen.

Disclaimer: I've updated the tests, but I've not tried running the update code against a real interactive piece of content.