Container expiration policies enabled by default

What does this MR do?

This updates the default value for container_expiration_policies.enabled to be true, and updates the default value shown on the front end for container expiration policies older_than to be '90d'.

This will enable on the container expiration policy for new projects in the spirit of GitLab "enabled by default". Existing container expiration policies will not be affected.

Database

Migration output:

== 20200331220930 EnableContainerExpirationPoliciesByDefault: migrating =======
-- change_column_default(:container_expiration_policies, :enabled, true)
   -> 0.0053s
== 20200331220930 EnableContainerExpirationPoliciesByDefault: migrated (0.0125s)

Screenshots

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

Related #208268 (closed)