feat(usermodel): add repr method to User class

lumi2/usermodel.py

@@ -248,3 +248,10 @@ class User:
         if not User.is_valid_picture(picture):
             raise ValueError(f"Not a valid image: '{picture}'.")
         self.picture = picture
+
+
+    def __repr__(self):
+        repr_str = f'User("{self.username}", "{self.password_hash}", ' \
+                   f'"{self.email}", "{self.first_name}", "{self.last_name}", ' \
+                   f'"{self.display_name}", "{self.picture}")'
+        return repr_str