Add some features from pull request #99.

After seeing issue https://github.com/prose/prose/issues/251 when looking into why I couldn't save prose as a web app on my phone I decided I'd at least contribute the bare minimum.
That is support for a home screen icon, and a full screen view after pressing 'Add to home screen'. Thanks to @dhcole for the images he made in his request. Hopefully this'll get him to finish the cool work he started.