[mobile] Enable Add to Home Screen

[fabd]

For a better experience without the browser bar.

A first implementation doesn't use any fancy cache or "service workers" if possible. All it does is make sure the app icon launches the app without the browser chrome for slightly more screen estate, and ensure the user stays within the app instead of being sent to the browser.

Issues

• Most links in the app take the user out of the app and open the page into the browser.

Solutions

• add a handler on the document root to prevent default link behaviour and change the document url via Javascript. I haven't tested this and there may be other issues, I don't know.
• would Turbolinks help with this? I assume as part of the functionality they already handle the links (ie. intercept standard links and use javascript to set document location).

Also requires #2 for the app icon.