The original FieldSignature handled related_model as a special

argument, and a later version of that change got rid of that in favor of

a field attribute. The original idea was correct, as the equivalent

field attribute for Field classes required a model, whereas this

version represented a class path string.
This change restores this as its own attribute, and makes it easier to

construct a FieldSignature with a related model and field attributes

all at once. Diffing has been updated to factor this in as well.

Tested along with other pending changes. Related model handling worked.