Fix DjangoObjectType deprecation warnings

[ponas]

The following warnings were emitted from graphene-django:

DeprecationWarning: Creating a DjangoObjectType without either the `fields` or the `exclude` option is deprecated. Add an explicit `fields = '__all__'` option on DjangoObjectType CollectionObjectType to use all fields
DeprecationWarning: Defining `exclude_fields` is deprecated in favour of `exclude`.

Fixed by:

• explicitly setting fields = "__all__"
• setting exclude instead of exclude_fields

Closes #355