Fixes #110 by using a webview for 'view on site'

This incidentally adds the ability to leave feedback using the site form.
It doesn't replace doing this 'right' per #40, but it's an interesting
experiment. I'm certainly willing to pull it out.
Webview provided for view on site, why not messages too?

This doesn't solve #46, but is a useful short-term hack-around.
Merge pull request #133 from rfay/20141125_webview

Fixes #110 by using a webview for 'view on site'