macOS: in-app browser

[samuelclay]

As per https://forum.newsblur.com/t/macos-app-in-app-browser/10992/3. Let's push a new view for an in-app browser. Something we used to do a long time ago was support a right-ward swipe anywhere on the browser to go back to the story. That would be great and would make the in app browser a lower friction experience.

[samuelclay]

Let's also add the ability to choose different browsers: https://forum.newsblur.com/t/option-to-set-default-open-in-browser-for-macos-app/10989