Hide compose box based on stream posting policy

[gnprice]

When this Zulip feature is in effect:
https://zulip.com/help/stream-sending-policy
it may mean that the self-user wouldn't be able to send to a given stream. When that's the case, we should replace the compose box with a banner explaining this is the case.

In the API, this is ZulipStream.streamPostPolicy.

In zulip-mobile we have a version of this, but it only detects "announcement-only" streams, i.e. where the policy is StreamPostPolicy.administrators. (Because long ago that was the only version that existed.) Just recently I happened to see a user get affected by this on a stream where the policy was StreamPostPolicy.moderators and they weren't a moderator — the symptom (on zulip-mobile) was that the app tries to send and then the message just sits there with an outbox spinner forever. So that underlines that this feature is a meaningful one which we should be sure to implement (and do so completely).