Add descriptions to NodeNorm

[gaurav]

Some descriptions are now being generated in Babel (NCATSTranslator/Babel#109). We should add support for ingesting descriptions into the Redis database behind NodeNorm, and then presenting them in the API.

[gaurav]

@YaphetKG has implemented this, and it's up and running on RENCI Dev (e.g. https://nodenormalization-sri.renci.org/1.4/get_normalized_nodes?curie=MESH%3AD014867&curie=NCIT%3AC34373&conflate=true&drug_chemical_conflate=false&description=true). Closing.