✨(backend) add full_name short_name on User model

Following @sampaccoud's work on impress, add new fields to handle
user names in our application.

@sampaccoud preferred having a full and short names instead of
a basic first and last ones, to follow common good practices, and
avoid having frontend formating names (from my understanding).

Please see commit eee20033 on Impress.

src/backend/core/factories.py

@@ -23,6 +23,8 @@ class UserFactory(factory.django.DjangoModelFactory):
 
     sub = factory.Sequence(lambda n: f"user{n!s}")
     email = factory.Faker("email")
+    full_name = factory.Faker("name")
+    short_name = factory.Faker("first_name")
     language = factory.fuzzy.FuzzyChoice([lang[0] for lang in settings.LANGUAGES])
     password = make_password("password")