NewRefreshTokenSource returns a token source that obtains its initial token based on the provided config and the refresh token.

TitleBarRedirectURL is the OAuth2 redirect URL to use when the authorization code should be returned in the title bar of the browser, with the page text prompting the user to copy the code and paste it in the application.

ErrNoAuthCode is returned when Token() has not found any valid cached token and TokenSource does not have an AuthCode for getting a new token.

TokenSource is an implementation of oauth2.TokenSource.