Fix #11: Added Login with Google functionality that redirects to /stats page after Login

[biresh1929]

1. I connect to mongodb by connection url and paste it into .env file.
2. I also generate a secret key and place it in .env file.
3. Then I make an account on Google Developers Console where I set up OAuth consent screen and get Google client Id, secret and redirect url and place those on .env file.
4. Then in app.py file, I fetch the variables from the .env file and set the routes - /login/google and /authorize/google.
5. Then in index.html file, I create a "Login with Google" button.
6. In stats.html file, I make the template say Hello userId.