Add an include_name_to_use option to Person.additional_names

This commit also includes tests for the various name-related
methods.
Add a method to return a set of all known names for a Person
Add an Django admin command to merge two people

This replaces a series of manual manual steps which you used to
have to perform if you ended up with duplicate people in the
database, for example after a big data import where names are
ambiguous.

Thanks to Edmund von der Burg for reviewing this code,
providing a fix and suggestions for improvement.
Remove manual merge steps in favour of using the management command