Handle updated streams (`stream op: update` events)

[chrisbobbe]

Part of #135.

https://zulip.com/api/get-events#stream-update

Handling created/deleted streams (stream op: create / stream op: delete) is #181.

I expect this will involve updating both our store.streams and store.subscriptions data structures.

[chrisbobbe]

I'm not sure how best to represent the type of this event; see in the doc, for example, value: integer | boolean | string.

[chrisbobbe]

Ah: removing this from the "beta" milestone until we make that choice explicitly for this event type. From Greg's comment on the umbrella issue #135 (comment) :

I've given this the "beta" milestone, but there may be specific events we decide can wait for launch or even beyond. What's needed for beta is really just that we should look at the list and make any such choice explicitly.