people: Extract _calc_user_and_other_ids from get_recipients.
people: Extract _calc_user_and_other_ids from get_recipients.

We previously added support for showing the status_emoji to the PM
list, but we only supported individual PMs and not group PMs, this is
prep commit towards supporting group PMs.

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