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

Basically this is a response to the fact that every time my wife tested the app she was put off by the "view on site" requiring a separate login. A lot of work just to solve that! But it's essentially simple, except for having to save and access the cookie.

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. It also adds the ability to access messages in the same way. Well, we were providing the capability...