The logic of the partner merge wizard is to collect all relevant data
from source partners, write them to a destination partner, then remove
the sources.
This... doesn't work when the field in question has a UNIQUE
constraint (like github_login), because it's going to copy the value
from a source onto a dest which will blow the constraint, and so the
copy fails. In that case the user first has to *move over* the unique
field's value then they can use the wizard.
Just fix for the github login: take all sources, remove (and store)
their github logins, then write the login onto the dst.
An alternative would have been to *defer* the constraint, however:
* it only works on unique constraints, not unique indexes
* it requires the constraint to be declared DEFERRABLE
Closes#301
