Support new settings for restricting direct messages

[alya]

In zulip/zulip#24467, we plan to add new settings for restricting direct messages. It would be nice if the mobile app proactively communicated to the user when they are not allowed to send a direct message.

If we don't make mobile app changes to support this, the server will return an error when the user attempts to send a DM they aren't allowed to send. So at a minimum, we should make sure that any error messages we display to the user clearly explain what's goin on.

Web app / server issue:

• Add settings for restricting direct messages zulip#24467