✨(contacts) add notes & force full_name

We make the full name mandatory and add a field to
allow user to store personnal notes on the contact.

This also make the "base" contact not mandatory because
user may want to create new contacts out of the blue.

src/backend/core/api/client/serializers.py

@@ -17,10 +17,14 @@ class ContactSerializer(serializers.ModelSerializer):
             "base",
             "data",
             "full_name",
+            "notes",
             "owner",
             "short_name",
         ]
         read_only_fields = ["id", "owner"]
+        extra_kwargs = {
+            "base": {"required": False},
+        }
 
     def update(self, instance, validated_data):
         """Make "base" field readonly but only for update/patch."""