Add auto_delete_at to environments

What does this MR do?

Adds an auto_delete_at column to the environments table which will be used to store a timestamp after which it's acceptable to delete that environment.

Related #296625 (closed)

Migration/rollback output

~/Development/gitlab/gitlab-development-kit/gitlab auto-delete-at* robotmay@Vetinari
❯ rake db:migrate                                                                                                                                                                   11:48:51
== 20210128114526 AddAutoDeleteAtToEnvironments: migrating ====================
-- add_column(:environments, :auto_delete_at, :datetime_with_timezone)
   -> 0.0009s
== 20210128114526 AddAutoDeleteAtToEnvironments: migrated (0.0029s) ===========

~/Development/gitlab/gitlab-development-kit/gitlab auto-delete-at* robotmay@Vetinari 6s
❯ rake db:rollback                                                                                                                                                                  11:49:16
== 20210128114526 AddAutoDeleteAtToEnvironments: reverting ====================
-- remove_column(:environments, :auto_delete_at, :datetime_with_timezone)
   -> 0.0008s
== 20210128114526 AddAutoDeleteAtToEnvironments: reverted (0.0052s) ===========

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