Notify user when DMing a guest, based on setting

[gnprice]

This is the zulip-flutter side of the following upcoming Zulip feature, with corresponding server/web and zulip-mobile issues:

• Add option to notify users when they are DMing a guest zulip#30078
• Notify users when they are DMing a guest based on setting zulip-mobile#5872

The feature is probably landing in Zulip Server 9.0, so it'd be good to implement soon. That means:

• consume that new setting from the server;
• when appropriate, show a banner above the compose box similar to the one shown in Zulip web and zulip-mobile.

Other related issues

• Similar feature but not directly related at a code level: Support new settings for restricting direct messages #791