refactor: Extract UserFilter.
refactor: Extract UserFilter.

This is setting us up for future commits.

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