[dagit] Bucket materializations in asset grid
A followup to https://dagster.phacility.com/D7487, which used materialization bucketing on the list view. This just does the same on the grid view.
Test Plan: View assets that have previous materializations for certain partitions, and assets that do not. Verify that the extra row appears at appropriate times in both grid and list views.
Reviewers: sandyryza, prha
Reviewed By: sandyryza
Differential Revision: https://dagster.phacility.com/D7503