add unique key constraint on Phenotype table

[raivivek]

Following from discussion on gitter.

[raivivek]

@gedankenstuecke I am going to assume that since we have a validation step at the controller stage there will be no duplicate entries in the Phenotype table right now. If not, then one may have to operate on the table and merge duplicate entries...

[gedankenstuecke]

@vivekiitkgp I think there were like 2-3 duplicate phenotypes which would have to be manually dealt with before. Thanks for the work on it so far!

[tsujigiri]

I'd prefer that we deal with it through a reviewable and testable migration over fiddeling around with it manually. ;)

[gedankenstuecke]

Fair enough :p

[raivivek]

@tsujigiri On it!