Species uniqueness constraint

Add a uniqueness constraint on Species objects:
    `(instance, common_name, genus, species, cultivar, other_part_of_name)` should be unique.

But first we have to eliminate existing duplicates from two sources:
1) Our own data -- in 6 cases, two OTM codes have the same species info
2) Past mistakes in custom imports

So there are two data migrations to eliminate the duplicates, and one schema migration to add the constraint.