Do planning for galaxy_ng api/v1 option(s) to address user access to namespaces

 
Namespace updates per PR review:

1. A proper RBAC implementation for legacy namespace
2. Replacement for the username==legacy_namespace.name logic to determine if a user owns a namespace
3. Some kind of system to migrate the existing users from galaxy into galaxy_ng so we don't have to rely on the user's github username matching one of the namespaces