[dagit] Pad the bottom of the Gantt chart to avoid last row of steps behind search box #4389

Summary: Just some missing padding on the container

Test Plan: Verified that it looks good, scrolls correctly, padding does not cause N = N + 100 forever issue with viewport sizing

Reviewers: dish

Reviewed By: dish

Differential Revision: https://dagster.phacility.com/D9100