[dagit] Clean up excess spinners on Runs page

Summary:
A handful of changes to reduce spinner thrash on the Runs page and elsewhere:

• For the tab counts, initially show an em-dash instead of a spinner while we're loading the counts.
• On subsequent refetches, do not show spinners in place of the numbers. Just leave the numbers alone.
• Don't show the spinner on the TokenizingField (filter input) unless it's open. If the user doesn't interact with it, it's not useful to know that it's loading.

This leaves only the loading bar, which we probably need to replace more broadly across Dagit anyway.

Hopefully this is an improvement over the existing spinnerfest.

Leaving the associated task open while we figure out repro steps for the noted perf issue.

Test Plan: View Runs page, verify rendering as described above. Let countdown reload data, verify same.

Reviewers: bengotow, catherinewu, schrockn

Reviewed By: bengotow

Differential Revision: https://dagster.phacility.com/D6169