Google credential documentation additions (OAuth, consent screen)

[andrewmilligan]

Some Google credential additions suggested by @zstumgoren

1. Explain why users are setting up OAuth (again) in https://harvester.readthedocs.io/en/latest/google_credentials/#oauth-authentication E.g.

   We recommend setting up your Harvester instance so that users must log in before they can add data. This step will set up the application in way that anyone who you’ve shared the Google Sheet with will be able to log into Harvester and perform data entry.

2. Explain what a Consent screen actually is here https://harvester.readthedocs.io/en/latest/google_credentials/#consent-screen E.g.

   A consent screen is a web form that users will see when they attempt to log into a form on the Harvester site. This screen will ask users to choose a gmail account with which to log in (if they have multiple accounts, they should choose the one that the project’s Sheet was shared with).