Web Push subscription and registration

[zealsprince]

Ergo delivers pushes itself; no Orbit component sits in the path. The client's half: negotiate draft/webpush, obtain a subscription from the service worker's Push API, and hand the endpoint plus key material to the server via WEBPUSH REGISTER, re-registering when the browser rotates the subscription.

Spec

• Push - the four client steps

Acceptance criteria

• With the cap available and permission granted, the subscription registers with the server
• A browser-rotated subscription re-registers without user action
• Logout deregisters the subscription
• Against a server without the cap, no push UI appears and nothing errors

Blocked by #30 and the PWA setup ticket.