Streaming API

Clients should have a way to tap in to some sort of streaming API to get updates from the server in realtime.

I've had good luck with the [EventSource](https://caniuse.com/eventsource) API on both the client and server side of it. We should define a list of event types that map to the different Microsub actions, so that there's a way for the server to push new events to the client. Things like:

* channel name and unread count when a post is added to the channel
* new entries added to a channel (maybe only when a channel is viewed? should all posts be pushed to the client? should the client request entries per channel separately?)
* what else?

Start by defining the EventSource-based streaming API and the event types corresponding to Microsub actions. Resolve whether channel updates and new entries are pushed globally, only for viewed channels, or requested per channel; done means the scope and event list are specified clearly enough for clients and servers to implement.

Written by the indexing model from the issue text.