Single session throughout deployment

[Shivam010]

As the Google Oauth client is set in a Global variable. The deployment can only process or work with a single user if multiple users are trying to use at the same time, the user arriving at the last will only be able to use the service, others will get the error as the deployment now has the Google drive access for the last user's account.

Note: As the project was just meant to solve my simple problem of rendering template using sheet's data, I never changed it. It's just one of my problem-solving experiments.