Person Type screen
A user should be able to re-use person types per product.
Currently, as things are, users will keep creating new person types for each product
To address this, we need to create a separate screen for browsing all post types of type "person type".
Also where it currently says `Add Person Type` we will need two buttons, one for creating and one for linking to existing person type.
This behaviour is seen in some of the other extensions as a feature. E.g. in the product data metabox, you click Remove but it's not actually removed, it's unlinked from that product, but there is a separate screen for that post type (e.g. person types) where you can actually delete it for real.
The behaviour is similar to that of resources