[dagit] Fix Assets scrolling
Summary: I broke scrolling on the assets pages when introducing the top nav. It'd be good to have a shared Page abstraction here, but this fixes the immediate issue.
Test Plan: View /assets and individual asset pages. Shrink window height, verify that scrollbar is functional.
Reviewers: bengotow
Reviewed By: bengotow
Differential Revision: https://dagster.phacility.com/D4689