[dagit] Some loading-state cleanup
Some minor clean up in LeftNavSimple and the RunRoot header, especially for loading state.
- Fix Dagster logo and version alignment.
- Remove spinner from RunRoot header, it's a bit jarring and unhelpful, and its alignment is wrong.
Test Plan: View Dagit, verify above.
Reviewed By: bengotow
Differential Revision: https://dagster.phacility.com/D6447