[dagit] Split apart Instance Health query

Summary:
Currently InstanceHealthQuery does some heavy fetching for Schedules, even if the user doesn't click the "Schedules" tab.

Break the page and its query apart.

Test Plan: View /instance, then each tab. Verify that querying and refreshing work properly.

Reviewers: prha, bengotow, dgibson

Reviewed By: dgibson

Differential Revision: https://dagster.phacility.com/D7699