Offer a stream list, for subscribed streams

[gnprice]

There should be a way to navigate to a desired stream, out of any of the streams you're subscribed to.

(There should also be a way to get to streams you're not currently subscribed to, and subscribe to them. But I'll file a separate issue for that; -> #188.)

This should probably look like just a scrollable list of subscribed streams, much like we have in web's left sidebar and in zulip-mobile's streams tab.

For where to put it in the UI: like with recent DM conversations #119, for now let's give this its own screen and navigate to that screen with a new button added to HomePage. Once we have this and #119 and perhaps an inbox view #117, then we'll be well set up to see about fitting them all into a nicer nav scheme than that.

[gnprice]

Out of scope for this issue: this screen might ultimately have a feature to show the topics within a stream, by tapping something on the stream to expand it — akin to web's left sidebar.

That will be a post-alpha (probably post-beta) task, so we'll track it separately. (→#412.) For the alpha, the goal is just to have a way to navigate to a stream when there aren't unreads or very recent messages in it.