Add exclusion constraints on ElementPath ordering

Closes #762 (closed)

This adds two exclusion constraints to ElementPath to ensure that an element cannot be at two different orderings on the same parent and that there cannot be two elements with the same ordering on the same parent. This is best tested with !1982 (merged) as the command will allow cleaning up the existing paths so that the migration can pass.

The migration took one hour to apply on 46 million paths locally.