[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