Add disease chemical data from CTD

[kshefchek]

Currently we're ingesting these and indexing them in solr, but they're not making it to our site. We'll need to add a /disease/{id}/chemicals endpoint. Note that we're already getting the chemical counts linked to disease, eg https://api.monarchinitiative.org/api/bioentity/disease/MONDO:0018020?fetch_objects=false&unselect_evidence=true&exclude_automatic_assertions=true&use_compact_associations=false&get_association_counts=true&rows=0

cc @diatomsRcool