MM-14446: consider subpath when evaluating if url is internal (#946)

* MM-14446: consider subpath when evaluating if url is internal

When clicking on an URL with `target=_blank`, the webview decides if it should launch an external browser or a new window within the Electron application. Update this logic to consider the application's configured subpath so as to treat links outside the subpath but on the same domain as external.

* fix licensing on new file

* fix .eslintrc.json indentation

* tweak header eslint rules for specific files

test/specs/utils/util_test.js

@@ -0,0 +1,47 @@
+// Copyright (c) 2016-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+'use strict';
+
+import url from 'url';
+import assert from 'assert';
+
+import Utils from '../../../src/utils/util';
+
+describe('Utils', () => {
+  describe('isInternalURL', () => {
+    it('should be false for different hosts', () => {
+      const currentURL = url.parse('http://localhost/team/channel1');
+      const targetURL = url.parse('http://example.com/team/channel2');
+      const basename = '/';
+      assert.equal(Utils.isInternalURL(targetURL, currentURL, basename), false);
+    });
+
+    it('should be false for same hosts, non-matching basename', () => {
+      const currentURL = url.parse('http://localhost/subpath/team/channel1');
+      const targetURL = url.parse('http://localhost/team/channel2');
+      const basename = '/subpath';
+      assert.equal(Utils.isInternalURL(targetURL, currentURL, basename), false);
+    });
+
+    it('should be true for same hosts, matching basename', () => {
+      const currentURL = url.parse('http://localhost/subpath/team/channel1');
+      const targetURL = url.parse('http://localhost/subpath/team/channel2');
+      const basename = '/subpath';
+      assert.equal(Utils.isInternalURL(targetURL, currentURL, basename), true);
+    });
+
+    it('should be true for same hosts, default basename', () => {
+      const currentURL = url.parse('http://localhost/team/channel1');
+      const targetURL = url.parse('http://localhost/team/channel2');
+      const basename = '/';
+      assert.equal(Utils.isInternalURL(targetURL, currentURL, basename), true);
+    });
+
+    it('should be true for same hosts, default basename, empty target path', () => {
+      const currentURL = url.parse('http://localhost/team/channel1');
+      const targetURL = url.parse('http://localhost/');
+      const basename = '/';
+      assert.equal(Utils.isInternalURL(targetURL, currentURL, basename), true);
+    });
+  });
+});