[dagit] Asset: swap "By Partition" and "By Timestamp" default
Summary: For partition set assets, show "By Partition" by default, instead of "By Timestamp".
Test Plan: View a partitioned asset, verify default.
Reviewers: sandyryza, bengotow, prha
Reviewed By: prha
Differential Revision: https://dagster.phacility.com/D7344