Change unique constraint on OuIdentifiers

OuIdentifiers used to be restricted to the name+value combination to
make sure that no two units could have the same legacy_stedkode. This
becomes problematic when two units have the same short_name.

This makes it possible for two units to have the same value for the same
OuIdentifier name, and adds a constraint that makes it impossible for
each unit to have more than one OuIdentifier with the same 'name', i.e
multiple legacy_stedkode for one unit.