[dagit] Restore nav height fix for Safari
Looks like I accidentally dropped the fix from https://github.com/dagster-io/dagster/commit/585b2130ee033fcbc666dac6dbd3b283c6f09f95 when creating TopNav. Restore it.
Test Plan: View Runs page in Safari, verify that the nav height stays correct. Shrink the window horizontally, verify that tabs wrap to the second line.
Reviewers: bengotow, alangenfeld, prha, sashank
Reviewed By: alangenfeld
Differential Revision: https://dagster.phacility.com/D4790