Add GAO synonyms

Using the output from @opendata’s Legal Synonyms project
(https://github.com/opendata/Legal-Synonyms/), specifically the
analysis of the GAO’s legal thesaurus. This certainly isn’t perfect,
but it’s a far sight better than having no legal thesaurus. This is
going to result in some unrelated results, but after doing some
comparisons, I think it’s an improvement.

Per #550.