Resolve "undefined method `allow_mfa_for_subgroups' for nil:NilClass after upgrading to 13.5.1 (8ee0746f54c)"

What does this MR do?

There were reports on groups created and throwing 500 error on edit page (/groups/name/-/edit). It happened when group was created via admin panel - it was caused by lack of namespace_settings record, it was not created when creating group via admin panel, as it does not use service as api endpoint and main interface controller.

This fixes it plus adds migration to fix inconsistencies in the data.

Screenshots (strongly suggested)

database:

== 20201104124300 EnsureNamespaceSettingsCreation: migrating ==================
== 20201104124300 EnsureNamespaceSettingsCreation: migrated (0.0582s) =========

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 to #273380 (closed)