Refactor NamespaceLicense

[Taucher2003]

Currently a license has to be attached to a namespace. On self-managed EE the license should be attached to the whole instance and not to individual namespaces.

• Rename NamespaceLicense to License
• Rename table namespace_licenses to licenses
• Make association in License to the Namespace optional
• Create the cloud extension to add a validation to License that namespace is present
• Add a validation that namespace is absent unless the cloud extension is active
• Add the create and delete mutations to the ee GraphQL extension.
• Add a Query.licenses and Query.current_license field to the ee GraphQL extension.
• Make the namespace argument in License.current and License.load_license optional
• Move EE::Namespaces::Members::InviteService#validate_user_limit! to the cloud extension
• Add a ee extension for Users::CreateService and Users::RegisterService to validate the user limit unless the cloud extension is active