Initial work on updating other associated object data on change

If a person is assigned to a project, that project should also be
assigned to the person. When people are removed from a project, that
project should be removed from those people. Etc...
Use the post type for the IDs storage key
Set the people content type to wsuwp_uc_person, singular
Remove the duplicate wsuwp_uc_ prefix from post meta keys
Maintain the association of objects as they are cross assigned
Our key is sometimes 0, which doesn't pass the if() check