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.

[gnprice]

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

Update: a couple of months after this comment, we added UserSettingsUpdateEvent, which has a very similar pattern in the API:
https://zulip.com/api/get-events#user_settings-update
and we worked out a pattern for handling that which we're pretty happy with. So for adding StreamUpdateEvent, let's follow that pattern.

To see all the different pieces of that pattern, see commits 9cb2e4d and 4bcbffd (both from #261). I think a good PR for this issue will look a lot like those two commits.

[sm-sayedi]

I am going to assign this to myself as it is needed for #674.