Refactor serializers to handle default value for `bar`
With django-rest-framework 3.8, serializers fields with "default" and "read_only" behave differently from before.
As a result, we have to add a function
save() for each serializer that has a field:
bar = serializers.PrimaryKeyRelatedField(read_only=True, default=CurrentBarCreateOnlyDefault())
so that the field is correctly populated.
Suggested approach: create a mixin
CurrentBarOnCreateMixin that implements that logic.