Add migration to convert the external document many-to-many relationship to a one-to-one.

Modify the external document mixin to define a basic one-to-many nested record.

Clean up the unique location (within a record) validations and add a translation (it was without an error msg at all).
Merge pull request #685 from hudmol/make-external-documents-one-to-many

Make external documents a simple one-to-many nested record