Support Web Push protocol

[XVilka]

To send the notifications from the server to the page:

• https://developer.mozilla.org/en-US/docs/Web/API/Push_API
• https://tools.ietf.org/html/draft-ietf-webpush-protocol-12
• https://tools.ietf.org/html/draft-ietf-webpush-encryption-09
• https://tools.ietf.org/html/draft-ietf-httpbis-encryption-encoding-09
• https://tools.ietf.org/html/draft-ietf-webpush-vapid-04
• https://w3c.github.io/push-api/

[roddyyaga]

I've been looking into this here. The main thing blocking sending Web Push notifications from OCaml is support for signing JWT tokens with ECSDA. But I think regardless of that, it probably wouldn't make much sense to implement it in a server like Opium anyway -- you send Web Push notifications by making requests to services owned by browser manufacturers, not by communicating with clients directly.