compose: Don't show compose box for private, unsubscribed channels#1652

chrisbobbe wants to merge 2 commits intozulip:mainzulip/zulip-flutter:mainfrom

chrisbobbe:pr-hide-compose-private-unsubscribedchrisbobbe/zulip-flutter:pr-hide-compose-private-unsubscribedCopy head branch name to clipboard