Support setting streams announcement-only, and other post policies

[gnprice]

@rishig mentioned that some recently-added features that show up when creating a stream haven't been implemented for mobile.

• One is "private with shared history": Make a stream "private with shared history" #2683
• I don't know what the others are.

@rishig , would you comment with the others you know about?

[timabbott]

The other is "announcement only".

[jainkuniya]

on it

@zulipbot claim

[gnprice]

Recent (unreleased, post-2.1) server versions no longer have announcement_only and instead have a stream_post_policy enum. Discussion here:
https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/Add.20functionality.20to.20create.20announcement.20streams/near/855688
and docs here:
https://zulipchat.com/api/add-subscriptions