Support "support" stream type with topic-specific permissions

[alya]

In zulip/zulip#19434, we plan to make it possible to restrict which topics in a stream a user can view and send messages to. Users with restricted permissions:

a. Can start a new topic, and have regular permissions for that topic (read, post, etc.).
b. Cannot view or send messages in other topics.
c. Only see their own topics in auto-complete suggestions in the compose box.
d. Get an error if trying to send to somebody else's topic. It's probably OK that this leaks information about what other topics exist.

We should make sure the mobile apps can support these restrictions.

Web app / server issue:

• Create "support" channel type with topic-specific permissions zulip#19434