Add migration to cleanup audit_events backfill

Add post deployment migration that will clean up any incomplete jobs
from the partitioned audit_events backfill migration, and copy any
missing data.

changelogs/unreleased/229903-audit-events-post-backfill-migration.yml

+---
+title: Migration to cleanup after partitioned audit_events backfill
+merge_request: 41605
+author:
+type: added

db/post_migrate/20200904174901_backfill_cleanup_for_partitioned_audit_events.rb

+# frozen_string_literal: true
+
+class BackfillCleanupForPartitionedAuditEvents < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::PartitioningMigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    finalize_backfilling_partitioned_table :audit_events
+  end
+
+  def down
+    # no op
+  end
+end

db/schema_migrations/20200904174901

+739a4d5374ae2be419c5a2f8c1427f85a0b76c90233918b7b43f1c166ab06cec
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