Only register top-level namespaces while we bootstrap authorization for sub-spacing

[jhiemstrawisc]

Right now, nothing prevents me from registering /jhiemstra as a namespace and someone else from registering /jhiemstra/foo as a namespace. Eventually we should make sure this kind of sub-spacing is authorized by the entity that owns /jhiemstra.

For now, while we bootstrap up to that kind of authorization, we should at least enforce that if /jhiemstra is already registered, /jhiemstra/foo is restricted. Likewise, if /jhiemstra/foo is already registered, we should restrict registration of the parent space /jhiemstra.

[jhiemstrawisc]

Closed by PR #460