Issue: Namespace Tag Duplication

A namespace's BB-Code Tag is only validated when the namespace is first created. Entering an invalid or duplicate value on subsequent edits can result in a malformed BB-Code name, a BB-Code conflict with another namespace, a namespace like Island or Header getting a BB-Code, and/or a BB-Code overwriting a vBulletin default BB-Code (such as NOPARSE, or URL, which needless to say would be bad).