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

.eslintrc.js

@@ -1,10 +1,26 @@
 module.exports = {
     extends: ['airbnb', 'airbnb-typescript', 'prettier'],
-    plugins: ['@typescript-eslint', 'no-relative-import-paths', 'prettier'],
+    plugins: ['@typescript-eslint', 'no-relative-import-paths', 'prettier', 'header'],
     parserOptions: {
         project: './tsconfig.json',
     },
     rules: {
+        'header/header': [
+            'error',
+            'block', // comment type
+            [ // expected file header comment
+                // the spaces are important, otherwise, the rule will mark it as an error
+                '',
+                ' * Copyright (C) Contributors to the Suwayomi project',
+                ' *',
+                ' * 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/.',
+                ' ',
+            ],
+            2, // number of new lines after comment
+        ],
+
         'prettier/prettier': 'error',
 
         'no-plusplus': ['error', { allowForLoopAfterthoughts: true }],