Change template manifest start_url to dot to make it react-router fri…

Created by: foochu

The current template has public/manifest.json start_url set to "./index.html". This causes a problem with folks using client side routing since the home screen shortcut points to "https://myapp/index.html" and index.html does not match anything. I'd suggest we change start_url to "." for consistency with the dev server etc. This makes the shortcut point to "https://myapp" instead which is the "standard" form most people use to access these apps. I hope this helps.