Support non-local SBH fetches

[cjmyers]

When you are examining a collection on a SBH instance, it may include URIs that are objects in other SBH instances. If you then attempt to get the SBOL for one of these URIs, it will tell you that this object is not present in the SBH instance attached to this SynBioHubFrontend. Therefore, it should attempt to get the SBOL from the instance based on the prefix of the URI of the object being requested.