Add expire_at to PipelineArtifact

Ref: #234042 (closed)

What does this MR do?

This MR adds a new column to our new model introduced in !37969 (merged).

Why are doing this?

In order to depressurize the storage capacity we need to set up an expiration mechanism.

This approach is similar to how we handled this with our JobArtifact.

This will allow us to set up an expiration date for new artifacts being created at the pipeline level.

Database Review

Note: This is a brand new table which does not contain any data yet

Migration output

$ bin/rails db:migrate
== 20200810152043 AddExpireAtToCiPipelineArtifact: migrating ==================
-- add_column(:ci_pipeline_artifacts, :expire_at, :datetime_with_timezone)
   -> 0.0021s
== 20200810152043 AddExpireAtToCiPipelineArtifact: migrated (0.0021s) =========

Migration Rollback

$ bin/rails db:rollback
== 20200810152043 AddExpireAtToCiPipelineArtifact: reverting ==================
-- remove_column(:ci_pipeline_artifacts, :expire_at, :datetime_with_timezone)
   -> 0.0010s
== 20200810152043 AddExpireAtToCiPipelineArtifact: reverted (0.0033s) =========

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