Incrementing creation_counter breaks field equality

[mkjpryor-stfc]

These lines cannot be used to preserve ordering in forms, because Django's Field.__eq__ uses the creation_counter for equality:

markup_type_field.creation_counter = self.creation_counter + 1
rendered_field.creation_counter = self.creation_counter + 2

Unless the MarkupField is the last field in a model class, this causes a collision of creation_counter with the fields that come after. This breaks the Field equality check and hence any code that relies on field equality. For instance, I came across this bug when using django-concurrency, which relies on field equality to detect the version field.