pm_list_data: Rename display_message => display_object.
pm_list_data: Rename display_message => display_object.

We try to keep the term "message" in variable names reserved for
things that are actually Zulip messages.

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