Support SSO with Office.js in Outlook add-in, if Outlook URL is custom/vanity URL

Current behavior

Redirect URL mismatch error when using SSO with Office.js in Outlook add-in, if Outlook URL is custom/vanity URL.

Feature 

When URL for Outlook is a custom/vanity URL, then office-js should be able to fetch access tokens using SSO in the outlook add-in.

Steps to check current behavior

1. Change the outlook URL to some custom/vanity URL.
2. Follow the steps provided in Enable single sign-on (SSO) in an Office Add-in
3. Use code mentioned in Add client-side code of the same document to fetch access token using SSO.