refactor: Change highlight_person to person_highlighter in search.
refactor: Change highlight_person to person_highlighter in search.

This is a prep commit for #20494.

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