[dagit] Reduce some extra renders in Schedules table

Summary: While investigating a bug, I spotted some extra unnecessary renders during idle time in the Schedules table. Use React.memo and useMemo to trim the excess.

Test Plan: View /instance/schedules, use React profiler to verify that there are no excess renders while idling.

Reviewers: bengotow, prha, dgibson

Reviewed By: dgibson

Differential Revision: https://dagster.phacility.com/D5935