🔒️(back) remove pagination and limit to 5 for user list endpoint

The user list endpoint does not use anymore a pagination, the results is
directly return in a list and the max results returned is limited to 5.
In order to modify this limit the settings API_USERS_LIST_LIMIT is
used.

src/backend/impress/settings.py

@@ -604,6 +604,12 @@ class Base(Configuration):
         },
     }
 
+    API_USERS_LIST_LIMIT = values.PositiveIntegerValue(
+        default=5,
+        environ_name="API_USERS_LIST_LIMIT",
+        environ_prefix=None,
+    )
+
     # pylint: disable=invalid-name
     @property
     def ENVIRONMENT(self):