🤡(demo) generate dummy user

To search users we need to have some dummy data users.
We add some dummy users when we run the `make demo`
command.

src/backend/demo/defaults.py

@@ -0,0 +1,5 @@
+"""Parameters that define how the demo site will be built."""
+
+NB_OBJECTS = {
+    "users": 100,
+}