Make import/extensions eslint rule an exception in ES6 files

[MAILPOET-1139]

.eslintrc.es6.json

@@ -17,6 +17,7 @@
     // Exceptions
     "comma-dangle": ["error", "always-multiline"],
     "no-script-url": 0,
+    "import/extensions": ["error", "ignorePackages"], // without this rule we would have to import jQuery like this: `import jQuery from 'jquery'/index.js;`
     // Temporary
     "import/no-amd": 0,
     "react/no-multi-comp": 0,
@@ -40,7 +41,6 @@
     "jsx-a11y/alt-text": 0,
     "func-names": 0,
     "object-shorthand": 0,
-    "import/extensions": 0,
     "import/no-extraneous-dependencies": 0,
     "camelcase": 0,
     "eqeqeq": 0,