Fixes search history facets. Closes #380

This removes the previous hack that was causing the search history
problems and replaces it with a shim on Blacklight's facet labelling.

This also fixes an issue with testing where the ntriples mime-type
wasn't registered
Adds documentation on facet hack.
Merge pull request #384 from OregonDigital/feature/SearchHistoryURIs

Fixes search history facets