Media embeds, take three

:eyeglasses: @spladug @chromakode

Hello hello and welcome to another episode of liveupdate media embeds!

This takes into account feedback from the previous PR and has a good deal of improvements. Primarily:

* `MediaByURL` is now used to fetch cached embeds across updates.
* Embedding/scraping logic has been gathered into one module, `media_embeds`
* Embeds are only rendered if the URL is on its own line (and additionally updates now render as block.)
* `liveupdate_q` has been clarified as the `liveupdate_scraper_q`

Still "outstanding": We haven't solved the redditmedia herd problem, but I'm wondering if we should cross that bridge when we get to it. It's a sticky problem, and since it's under redditmedia, that would mean it's going to be served through our CDN infrastructure anyhow, right? There's additionally a bit of kludginess when it comes to the MediaByURL caching, as you'll see inline. I'd be happy to volunteer to clean that up down the line, but I don't think it should block us from this work.

Thoughts appreciated, thanks for feedback on the previous PR. Keep in mind that you'll need this pull request to allow differing widths to be in your working copy of core if you want to test.