compose_closed: Extract get recipient label helper.
compose_closed: Extract get recipient label helper.

This makes it possible to reuse this logic.

andersk/zulip,andersk/zulip,andersk/zulip,zulip/zulip,kou/zulip,rht/zulip,zulip/zulip,andersk/zulip,kou/zulip,zulip/zulip,andersk/zulip,rht/zulip,zulip/zulip,rht/zulip,zulip/zulip,kou/zulip,rht/zulip,rht/zulip,kou/zulip,andersk/zulip,kou/zulip,zulip/zulip,rht/zulip,zulip/zulip,kou/zulip,andersk/zulip,rht/zulip,kou/zulip