[dagit] Allow termination of entire backfill
Allow bulk termination of a backfill from the partition set page.
When the view is filtered to a specific backfill tag that still has unfinished runs, a "Terminate" button is rendered. Clicking the button passes the list of unfinished runs to the termination dialog, which then attempts to terminate the runs on confirmation.
Test Plan: Kick off a backfill. Verify that the Terminate button appears, click it. Verify correct termination workflow.
Reviewers: sandyryza, catherinewu, bengotow
Reviewed By: bengotow
Differential Revision: https://dagster.phacility.com/D5786