Add migration to cleanup audit_events backfill

What does this MR do?

Migration to call the partitioning migration helper that cleans up after the background migration used to backfill the audit_events table.

For details on that migration helper, see: !35201 (merged) and related issue #229903 (closed)

On GitLab.com, all the background jobs were completed successfully, so this won't have any missed records to copy.

 select status, count(*) from background_migration_jobs group by 1;
 status | count
--------+-------
      1 |  6909

Migration Output

up

rails db:migrate:up VERSION=20200904174901
== 20200904174901 BackfillCleanupForPartitionedAuditEvents: migrating =========
-- transaction_open?()
   -> 0.0000s
-- table_exists?("audit_events_part_5fc467ac26")
   -> 0.0004s
-- execute("VACUUM FREEZE ANALYZE audit_events_part_5fc467ac26")
   -> 0.0149s
== 20200904174901 BackfillCleanupForPartitionedAuditEvents: migrated (0.0750s)

down

rails db:migrate:down VERSION=20200904174901
== 20200904174901 BackfillCleanupForPartitionedAuditEvents: reverting =========
== 20200904174901 BackfillCleanupForPartitionedAuditEvents: reverted (0.0000s)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team