[dagit] Fix clicks on loading overlay
Fix clicks on the loading overlays in Dagit playground. The overlay should observe all pointer events, not allow them through.
Test Plan: View Playground, force loading overlay to appear. Verify that I can't click anything underneath it.
Reviewers: bengotow, sashank, prha
Reviewed By: sashank
Differential Revision: https://dagster.phacility.com/D5359