[dagit] Top-level pages for Schedules, Sensors
Summary: Create AllSchedulesRoot and AllSensorsRoot, reusing display code from the existing Instance tabs. When the nav changes are launched, we can remove those tabs from that page and refactor this a bit.
Test Plan: Enable simple nav, click on "Schedules" and "Sensors". Verify proper rendering and behavior. Navigate to individual schedules and sensors, verify that the nav items are appropriately highlighted.
Reviewers: bengotow, prha, dgibson, johann
Reviewed By: dgibson
Differential Revision: https://dagster.phacility.com/D6328