Feature/person model updates

[XanderVertegaal]

This PR updates the Person model (resolves #11) and outfactors LetterActionDate to the core module (resolves #18).

Note: this branch is based on feature/separate-apps, which should be merged first.

In more detail:

• The unique=True on the PersonName.person field has been replaced by a UniqueConstraint. Adding multiple names to one person would not work properly before, but now it does. This also removes a recurring warning about having ForeignKey(unique=True), which Django does not like. Person.__str__() has been updated appropriately to deal with multiple names.

• Tests have been added for the display of Person names, but I am not sure if the way I did it is the way to go. (See comments below; input is appreciated.)

• Added models: Office (fancy name for what is essentially a 'job') and Occupation (for the relationship between a Person and an Office), PersonDateOfBirth and PersonDateOfDeath.

• The LetterActionDate model has been outfactored to an abstract model called LettercraftDate. The following models subclass it: LetterEventDate, PersonDateOfBirth, PersonDateOfDeath, Occupation.

• Running pytest displayed warnings about a deprecated setting in settings.py. Since we do not use it and it does not have a function anymore, I removed it.

[XanderVertegaal]

Thanks for the clear and extensive explanation. It's good to hear your thoughts on this. I will transfer a few fixtures I've added to the tests where they are used. There is no need to 'pollute' the global conftest.py with these.