[dagit] Add id to PartitionSet
Summary: While looking into #3503 I noticed an Apollo warning about PartitionSet needing an id field for better caching. This adds it, just duplicating the name field.
Test Plan: View partition set in Dagit, click to view different "Last X" slices, verify that the Apollo warning is gone.
Reviewers: bengotow, sandyryza, dgibson
Reviewed By: bengotow, dgibson
Differential Revision: https://dagster.phacility.com/D5839