[dagit] Restrict main content pane height
Summary: Restrict height of main content, now that the nav bar occupies 50px. This restores the schedules/sensors list in the left nav, and brings back bottom-fixed items like the timezone selector and playground run button.
Test Plan: Run dagit, verify that things are back to normal.
Reviewers: bengotow, dgibson, max
Reviewed By: dgibson
Differential Revision: https://dagster.phacility.com/D7236