Raise warning about stateful components

It would be useful in future refactoring

.eslintrc.json

@@ -9,6 +9,7 @@
         "no-underscore-dangle": 1,
         "react/jsx-indent": [2, 2],
         "react/jsx-indent-props": [2, 2],
-        "react/prefer-es6-class": 1
+        "react/prefer-es6-class": 1,
+        "react/no-set-state": 1
     }
 }