enhance User.__str__ with get_full_name

gestioncof/admin.py

@@ -211,9 +211,7 @@ class UserProfileAdmin(UserAdmin):
 # FIXME: This is absolutely horrible.
 def user_str(self):
     if self.first_name and self.last_name:
-        return "{} {} ({})".format(
-            self.first_name, self.last_name, self.username
-        )
+        return "{} ({})".format(self.get_full_name(), self.username)
     else:
         return self.username
 User.__str__ = user_str