reworked views and serializers to pass and take into account the "validation timestamp" to the model + refactored models to have a content validation method + adapted tests
# RENKAN API - Django REST Framework
## Configuration
The following setting can be defined:
RENKAN_USER_DISPLAY_FIELD
This defines the field that will be used in your user model to serialize the user information your API will send in the responses. ("username", "external_id"...) Will default to the USERNAME_FIELD of the User Model class.