Media rendering

[radiac]

I plan to address the three rendering issues in the roadmap by contributing a patch to bleach to support tag callables (in a similar way to their attribute callables).

That should allow us to efficiently convert and sanitise img tags and youtube embeds into a tags with the resource address on the href, specifying the media type as a data-yarr-media="(img|youtube)" attribute.

The javascript can then lazy-load them, improving render times and allowing future support for safely allowing arbitrary embeds.

(Issue opened in case it helps anyone who gets on to this before me)