Automatically remove outdated auto-deploy branches

This adds ReleaseTools::AutoDeploy::Cleanup, which can be used to automatically remove auto-deploy branches older than seven days. These branches are removed on the public projects, the security mirrors, and on dev.gitlab.org.

See gitlab-com/gl-infra/delivery#270 (closed) for more information.

TOD

• Run it on the projects to test things
  • This works, and all currently outdated branches have been removed
• Add unit tests
• Add Rake task to run the code
• Set up CI job to run the Rake task
• Set up CI schedule to run this job once a day