Option to "forget" previous named graphs

[cordawyn]

When adding a named graph on the SPARQL Endpoint (used for saving documents), users are allowed to provide invalid graph URIs. When saving documents to that location, the invalid URI persists throughout all SPARQL requests, which result in error responses from the SPARQL server.

Currently there is no option to get rid of those errors (except "hard-reloading" RDF Editor). I suggest either (a) validating graph URIs at graph URI creation time and/or (b) allowing users to clear/delete/edit the graph URIs to "fix" that.