Flysystem S3 for Drupal 8.x-1.0-alpha2+p1

* Merge branch '2770263/cors' into upstream-patches
* Merge branch '2779231/amazon-sns' into upstream-patches
* Merge branch '2772847/private-files' into upstream-patches
* Merge branch '2695695/generated-url' into upstream-patches
* Issue #2770657 by twistor, kalpaitch: If cname is specified the
bucket is still in the url
* Issue #2774877 by deviantintegral, twistor: testCreate() should
only test create() and not getExternalUrl()