[dagit] Give asset catalog list a new stacking context
Bug: the flat/folder button group on the asset list view has a z-index, given to it by blueprint, which puts it on top of the sliding left nav.
Fix this by giving the component a new stacking context.
Test Plan: Shrink the viewport width, navigate to assets. Open the left nav, verify that the buttons no longer show through it.
Reviewers: bengotow, prha
Reviewed By: prha
Differential Revision: https://dagster.phacility.com/D7566