[feature] WebPush push notification support

[dbkr]

Is your feature request related to a problem ?

Client that use the Mastodon WebPush API for notifications currently aren't able to send push notifications since GtS doesn't support the WebPush API.

Describe the solution you'd like.

GtS could implement the Mastodon push API (https://docs.joinmastodon.org/methods/push/) which would allow notifications to work in these clients.

I think this would involve something along the lines of:

• Adding endpoints to subscribe to notifications, get, edit and remove subscriptions
• Storing the current subscriptions in the database along with what they're subscribed to
• A component that sends pokes to the endpoint given by the client in the subscription payload
• Encrypting the payloads of those pokes with ECDH / AES

Resources:

• WebPush RFC: https://datatracker.ietf.org/doc/html/rfc8030
• WebPush encryption RFC: https://datatracker.ietf.org/doc/html/rfc8291
• Summary article: https://web.dev/push-notifications-web-push-protocol/

Describe alternatives you've considered.

GtS supports pull-based notifications currently, so any apps that use this by polling will be able to support notifications without issue.

Additional context.

This would fix #1139