streams api: Allow setting muted/pinned for other users (for support streams)

[showell]

See discussion here:

https://chat.zulip.org/#narrow/stream/127-integrations/subject/Mute.20channel/near/583514

For organizations doing support, it can make sense to auto-subscribe users to certain "support streams" in the API, and you may often want the streams to be pinned and muted by default. (Pin them so the users are aware of them, but mute them to avoid spam.)

We currently don't allow muting/pinning on behalf of users, so this would necessitate some backend changes, and we almost certainly want to restrict this to either subscription time and/or to people with stream-owner powers.

If this feature becomes popular, we would probably make it be part of the UI as well.

[zulipbot]

Hello @zulip/server-streams members, this issue was labeled with the "area: stream settings" label, so you may want to check it out!

[showell]

A closely related idea here is to make descriptions be pinned to the top of message streams, so for support streams you could suggest to users that they do things like mute/pin the streams (as well as explaining or linking to the protocols for how to ask good questions, etc.). Of course, if you don't pin a stream right away, there's the problem the users won't even know the stream exists. The original message to the stream sort of mitigates that, but maybe not enough.