Ensure o is available

Organization URL paths are scoped under o with URLs like https://gitlab.com/o/my-org/my-group. We need to ensure that the o namespace is available. When an Organization is being created, ensure the o namespace is not taken otherwise raise an error and instruct to rename the o User.

Any installation with a single Organization will (currently) use the global scope like /projects/new and not /o/my-org/projects/new so it does not need to worry about reserving o.