[MM-43656] Flush cookies to disk on finishing custom login (#2076)

* [MM-43656] Flush cookies to disk on finishing custom login

* Fixed a test

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

src/main/views/webContentEvents.test.js

@@ -25,6 +25,7 @@ jest.mock('electron', () => ({
             setWindowOpenHandler: jest.fn(),
         },
     })),
+    session: {},
 }));
 
 jest.mock('../allowProtocolDialog', () => ({}));
@@ -57,6 +58,10 @@ jest.mock('common/utils/url', () => ({
     isChannelExportUrl: jest.fn(),
 }));
 
+jest.mock('main/app/utils', () => ({
+    flushCookiesStore: jest.fn(),
+}));
+
 jest.mock('../../../electron-builder.json', () => ({
     protocols: [
         {