Provide a URL for resolving IRI fragments

[cmungall]

We would like to be able to resolve purls using amigo. Whilst this could be handled upstream, the easiest way to do this using the current OCLC system is to make URLs such as

http://amigo.geneontology.org/amigo/term/GO_0010458

valid

Alternatively, a different prefix may be OK, e.g.

http://amigo.geneontology.org/amigo/fragment/GO_0010458

The main thing is that by the time it reaches AmiGO, all that may be left is the fragment of the IRI

Hacky fix is s/_/:/. Robust fix is to have a registry of purl prefixes, e.g. in a json ld context.