[dagit] Avoid flashing progress bar in Loading
The Loading component currently shows a progress bar during the loading state of the supplied queryData. This often means a brief flash of the progress bar, then the fully rendered child of the Loading component. This can be jarring, and is often unnecessary.
Instead, wait a brief moment during loading before showing the progress bar. If the loading completes during that wait, the contents render without the progress bar flash. If the loading does not complete during the wait, show the progress bar as normal while the loading finishes.
Test Plan: View a bunch of pages on Dagit that use Loading. Verify that when loading is completed quickly, the progress bar does not flash.
Reviewers: bengotow, sashank, prha
Reviewed By: sashank
Differential Revision: https://dagster.phacility.com/D5410