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/components/util/LoadingPlaceholder.tsx

@@ -3,7 +3,8 @@
  *
  * 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/.
+ */
 
 import React from 'react';
 import CircularProgress from '@mui/material/CircularProgress';