Replace `Field.rel` by `Field.remote_field`

[Bruno Alla]

https://docs.djangoproject.com/en/dev/releases/1.9/#field-rel-changes

Field.rel and its methods and attributes have changed to match the related fields API. The Field.rel attribute is renamed to remote_field and many of its methods and attributes are either changed or renamed.

To note from Django 2.0 removals:

Field.remote_field.to attribute will be removed.

• Deprecated: 1.9
• Removed: 2.0

[Bruno Alla]

Looking more closely, this seems more complicated to implement in a robust fashion.