Handle concatenation of duplicated reactome_ids with '///'

[ChristopherEeles]

This is related to bhklab/PharmacoDI_snakemake_pipeline#9.

Once I implement this change in the database, the web-app needs to be able to parse duplicated reactome_ids but splitting on '///' before generating the links to Reactome.

Handling duplicate entries this way allows us to avoid complicated ERD changes (i.e., a join table from compound_annotation to reactome to handle 1:many relationships).