When authenticating Trello account with 'Trello for Gmail' Add-on, the user don't get a feedback the authentication was successful

Issue Summary

During the authentication process when the user is authorizing the 'Trello for Gmail' add-on to have access to their Trello account, the add-on doesn't provide any UI feedback informing the user that the authorization was a success, making them think there's an issue with the authorization process. 

Steps to Reproduce

1. Go to your Gmail account
2. Install the 'Trello for Gmail' add-on 
3. Open an email while the add-on is open
4. Click on 'Log in with Trello'
5. Authorize 'Trello for Gmail' to access your Trello account
6. Click on 'Confirm' to approve the access
7. User gets stuck on the pop-up window without a message informing them the authorization was a success

Expected Results

After confirming the authorization, the user should see a message informing them the authorization was a success, and inform them they can close the pop-up window, and can start using the add-on. 

Actual Results

The user gets stuck after clicking the 'Confirm' button and doesn't get any indication the authorization was a success. 

Workaround

After clicking the 'Confirm' button, the user can close the pop-up window, and refresh the page. Once that's done, they'll be able to use the 'Trello for Gmail' add-on as expected.