refactor: Use user_ids_string more internally.
refactor: Use user_ids_string more internally.

We want to avoid passing opaque messages around to
low-level functions that only need certain fields.

This sets us up to handle mark-as-unread messages
that aren't necessarily full messages.

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