✨(backend) add a is_first_connection flag to the User model

Backend part of #1796.

This changes allows to display an onboarding modal the first time that
the get_me() API view is called.
I originally tried to check if `User.last_login` was `None`, but it is
updated as soon as the user is logged, so I chose to create a flag on
the model.

src/backend/core/api/serializers.py

@@ -32,8 +32,21 @@ class UserSerializer(serializers.ModelSerializer):
 
     class Meta:
         model = models.User
-        fields = ["id", "email", "full_name", "short_name", "language"]
-        read_only_fields = ["id", "email", "full_name", "short_name"]
+        fields = [
+            "id",
+            "email",
+            "full_name",
+            "short_name",
+            "language",
+            "is_first_connection",
+        ]
+        read_only_fields = [
+            "id",
+            "email",
+            "full_name",
+            "short_name",
+            "is_first_connection",
+        ]
 
     def get_full_name(self, instance):
         """Return the full name of the user."""