☂️ Review unhandled events for beta

[gnprice]

This is a subset of:

• ☂️ Handle all Zulip events to keep our data up to date #135

For the beta milestone, we won't handle all the events in the Zulip API. But we should review the list of them and identify any that should be handled for the beta (and aren't already).

We'll then file issues for those, in the beta milestone.

[gnprice]

Done. See #135 for details.

It turns out we're nearly done with the events we'll want for the beta! The main such issue, which I just filed after this review, is:

• Handle muted streams and topics #346

which calls for handling the event types subscription, user_topic, and muted_topics which is a deprecated form of the latter.

There's also one we already had in the tracker:

• polls: Read-only support #165 (calling for handling submessage events)

though that's in the back half of the beta milestone, where I'd feel pretty OK with a beta that left it out.