Add "expiration_policy_completed_at" support in container repositories

🍍 Context

As described in #276479 (closed), the Container Registry will need a way to know if the last cleanup expiration policy execution was successfully completed or not.

The field we're going to add is also an additional way to track cleanup policy completion.

🔍 What does this MR do?

• Add a new column on the ContainerRepository model: expiration_policy_completed_at
• Set this new column only when a cleanup policy successfully completes.

Screenshots (strongly suggested)

n / a

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.
• [-] Tested in all supported browsers
• [-] 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

💿 Database

📈 Migration up

== 20201214084105 AddExpirationPolicyCompletedAtToContainerRepositories: migrating 
-- add_column(:container_repositories, :expiration_policy_completed_at, :datetime_with_timezone)
   -> 0.0021s
== 20201214084105 AddExpirationPolicyCompletedAtToContainerRepositories: migrated (0.0021s) 

📉 Migration down

== 20201214084105 AddExpirationPolicyCompletedAtToContainerRepositories: reverting 
-- remove_column(:container_repositories, :expiration_policy_completed_at)
   -> 0.0030s
== 20201214084105 AddExpirationPolicyCompletedAtToContainerRepositories: reverted (0.0030s)