Instead of hard coding CreatePriority, build a DAG

[oliverheywood451]

This is a follow up idea to issue #25

Right now create priority is just a hard coded number. It could be hard to understand where it comes from. Instead, we could hard code the blocking relationships (e.g. Buyer Users must be created before Buyers), use that to build a Directed Acyclic Graph, and then use that to determine create priority.

This is kinda cool, but low priority.