[dagit] Split apart Instance Health query
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