feat: handle the 'Register for Push' app function

[gyoshev]

Enables automatic handling of the Register For Push app function.

If the service worker is not located at /sw.min.js, or requires a different scope, these can be set through Leanplum.setWebPushOptions. This should have been done before handling a Register For Push function:

Leanplum.setWebPushOptions({
  serviceWorkerUrl: '/shop/sw.min.js',
  scope: '/shop/'
});