Warn when adding person with non-unique name

[Aldaviva]

It's easy for two admins to decide to add a person, and then we have duplicate entries. There is no enforced uniqueness check on the fullname field in case there really are two people with the same name.

Proposed solution
When saving a new person, have the UI check for fullname uniqueness and show a modal warning that lets the admin know about the possible collision. They can either go ahead and save anyway, or go back to editing the unsaved entry.