This is a full rewrite of the News widget in the administration UI, not

only porting it to the new Python and JavaScript widget classes, but

also moving all RSS feed loading client-side instead of server-side and

improving the display.
The new version no longer depends on the server-side RSS feed loading,

which didn't work if Review Board was behind a firewall or proxy server.

By loading client-side, we ensure that any user with Internet access can

see the feed.
The rendering has also improved. The News feed is now larger and is no

longer off to the side. It cleanly separates dates from headlines, and

adds more room around each row so that headlines aren't jumbled

together.
This makes use of the new reload support, allowing users to reload the

news if they choose to. There's no automatic, periodic reloading at this

point, but that may be added in a future change at the WidgetView

level.
The design of the widget is also generic, allowing us to potentially

reuse the JavaScript side with other RSS feeds, if we ever want to.

Tested the news widget against our actual RSS feed.
Tested that all links work.
Unit tests pass.