Support genes in multiple species in the database

[dondi]

In the general case, genes appear in multiple species. The current database design can associate a gene with only one species right now. For maximum generality, we need to move this to a many-to-many design:

• Create a new table called species with taxon ID as primary key, a species name, and potentially additional columns describing that species
• Remove these columns from the gene table
• Create a _genespecies table that associates genes with species

[kdahlquist]

Closing this because it is backlogged. We can re-open later when we support multiple species in the database.