Add migration to cleanup web_hook_logs backfill

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

changelogs/unreleased/323675-cleanup-web-hoo-logs-backfill-migration.yml

+---
+title: Migration to cleanup after partitioned web_hook_logs backfill
+merge_request: 57580
+author:
+type: other

db/post_migrate/20210326121537_backfill_cleanup_for_partitioned_web_hook_logs.rb

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

db/schema_migrations/20210326121537

+ac8063901a0de1f2e6d4fd5336f54bfca0bafa9d081066124ec9d2acf47c9b47
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