Added unique constraint for external id identifier for disambiguated org

[Camelia-Orcid]

This pool request has to be merged/deployed only after the coresponding database has been cleaned for duplicates.

[Camelia-Orcid]

Add Db constraint so only one external ID for the same type and identifier can be added for a disambiguated organization

[Camelia-Orcid]

Add Db constraint so only one external ID for the same type and identifier can be added for a disambiguated organization

[amontenegro]

@Camelia-Orcid this should be a draft right? because we should not merge it until we remove the duplicates from all envs right?