Basic implementation of homepage alert region

This creates a new sidebar region atop the front page of the site for special announcements, conditionally puts the markup into the homepage for said region, and performs basic styling.

The wireframes are mocked up to show a particular story or series, but this doesn't implement any special widgets for that — it's the usual basic region so sites are free to put a text widget or whatever else they want. Super flexible.

If you want some sort of special widget that links to a particular Post with a configurable "happening now" header, "latest" update box, etc (as shown in the wires), we can talk about that. It wouldn't be hard to implement except that the bit at the bottom about "the latest" implies the fancier post-with-mini-updates architecture that doesn't exist (yet?).