Set defaults for cleanup policy regexes

What does this MR do?

Container cleanup policies use a set of regexes to determine which tags to remove from a given container repository. Right now, there is no default, and blank values are allowed. The problem is, if a policy has a blank name_regex, no tags will qualify for deletion. If this is the case, the policy should not be valid. Since the name_regex column does allow NULL and blank values and we cannot change the existing records, we need to validate that the name_regex has a value only when the policy is enabled.

This MR adds:

1. Validation to ensure name_regex is not blank when enabled = true.
2. Default value for name_regex. The cleanup policies are created automatically when a new project is created. We don't want to continue to create policies with blank values that do nothing.
3. Change the default from enabled: true to enabled: false. We are disabling by default for now since adding a default regex will cause new policies to run and remove image tags. We want to be sure the user is aware tags may be deleted before enabling by default.

Database

Up migration

== 20201008144854 SetRegexDefaultsOnContainerExpirationPolicies: migrating ====
-- change_column_default(:container_expiration_policies, :name_regex, ".*")
   -> 0.0045s
-- change_column_default(:container_expiration_policies, :enabled, false)
   -> 0.0021s
== 20201008144854 SetRegexDefaultsOnContainerExpirationPolicies: migrated (0.0099s)

Down migration

== 20201008144854 SetRegexDefaultsOnContainerExpirationPolicies: reverting ====
-- change_column_default(:container_expiration_policies, :name_regex, nil)
   -> 0.0080s
-- change_column_default(:container_expiration_policies, :enabled, true)
   -> 0.0047s
== 20201008144854 SetRegexDefaultsOnContainerExpirationPolicies: reverted (0.0173s)

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