[activitypub-api] Server-sent Events for ActivityPub API

One of the user stories for the ActivityPub API task force is to enable
real-time updates for clients.

https://github.com/swicg/activitypub-api/issues/9

To help with this, I added a draft specification for server-sent events:

https://swicg.github.io/activitypub-api/sse

If you're interested, please review and provide comments on the GitHub
issue. I'd like to start a reference implementation soon.

Evan