CRUD to manage prefixes and namespaces

While working on #1114096: Add a non-Evoc UI for creating namespace mappings, the closest function I could find to add a namespace to rdfx is the private function _rdfx_save_vocabulary() to which you have to pass in a fake vocabulary so that it does not generate notices. I think the pure namespace+prefix storage logic could be taken out of _rdfx_save_vocabulary() so it can be used by _rdfx_save_vocabulary() and other modules too.