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.