* Exclude read_only=True fields from unique_together validation
* Test to ensure that unique_together validators can be removed
* Do not add uniquness_extra_kwargs when validators are explicitly declared.
* Add docs on validation in complex cases
* Add Meta.fields = '__all__' to serializer classes where required.
* Add explicit on_delete=models.CASCADE to ForeignKey fields.
* Use '.remote_field' and '.model' in preference to '.rel' and '.to' when inspecting model fields.
* Use new value_from_object in preference to internal _get_val_from_obj
When serializers has fields with something like `source=user.email`, the
uniqueness validator should check `email` field instead of `user`, cause
`user` is a model object.
In the automatically applied UniqueValidator, use the error message from
error_messages defined in the model instead of the generic default
UniqueValidator message.
This fixes#2878.
