Backport/19383/6.x (#19487)

* [eslint][tslint] require Apache 2.0 license headers

(cherry picked from commit 337fea30b9)

* [kbn-eslint-plugin-license-header] don't try to fix files with hashbangs

(cherry picked from commit db79f1501e)

* [eslint][tslint] autofix license headers

tasks/check_plugins.js

@@ -1,3 +1,22 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
 import fs from 'fs';
 import path from 'path';