[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