Feature/enforce license notice in each file via eslint rule (#304)

* [ESLint] Add "eslint-plugin-header" as dev dependency

* [ESLint::eslint-plugin-header] Setup rule

* [ESLint::eslint-plugin-header] Fix errors

src/react-app-env.d.ts

@@ -3,6 +3,7 @@
  *
  * This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
 
 /// <reference types="react-scripts" />