Self-updating manifest

[elviswolcott]

In many apps, the manifest needs to update immediately with the new patch. While season releases are announced ahead of time, it still may not be possible to coordinate an update with the new assets in a manner that works. Instead, there should be a package or code sample demonstrating how to check for manifest changes and pull the latest version.

Because Netlify cannot be used for pure asset hosting, a copy of the manifest and assets will have to be hosted elsewhere. Any consumers should keep a local copy and only attempt to download a new version after checking if it is newer to preserve bandwidth.

Any hosting code should be made available so that consumers that cannot cache the assets (i.e. we apps) can host the assets their self.

This can probably be accomplished with S3 for file hosting and Lambda for a simple version check API. By throwing Cloudflare (or some other CDN) in front most asset requests should be cached.