Handle created/deleted streams (`stream op: create` and `stream op: delete` events)

[chrisbobbe]

Part of #135.

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

Handling changed streams (stream op: update) is #182; there are some potentially tricky details to get right there.

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

[chrisbobbe]

I expect this to be pretty straightforward.

As a bonus, it will support adding helpers like createStream and deleteStream on PerAccountStoreTestExtension in test/model/test_store.dart, following the pattern of addUser there. Those helpers will support writing tests for logic that depends on the contents of store.streams, including quote-and-reply #116. (The "said" link is a message link, which might include a segment like /stream/243-mobile-team, for which we'll consult store.streams[streamId].name.)

Adding to the "Alpha" milestone to match #116.

[chrisbobbe]

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

I guess on "create", we don't want to add anything to store.subscriptions. The event doesn't carry details of the user's subscription to the stream, if they're even subscribed at all. If the user is subscribed, I assume the stream-create event will be followed by a subscription event.

On "delete", probably we do want to remove entries from store.subscriptions, like we do in zulip-mobile?