Enforce uniqueness on Variant model

[madprime]

The Variant model is assumed to have uniqueness with respect to the combination of chromosome, position, ref allele, and var allele. An error arose due to an accidental creation of duplicate Variants – potentially idiosyncratic – this shouldn't happen, because once it does, it will create ongoing errors.

The model should be updated to enforce uniqueness so database entries violating this uniqueness requirement can't be created in the first place.