Summary:
Enabled by default, the new checkbox is only visible to
users logged in with email/password, and controls whether it is possible
to log in to the same account via a Google account
(with matching email). When disabled, CognitoClient will refuse logins
from Google if a Grist account with the same email exists.
Test Plan:
Server and browser tests for setting flag. Manual tests to verify
Cognito doesn't allow signing in with Google when flag is disabled.
Reviewers: paulfitz
Reviewed By: paulfitz
Differential Revision: https://phab.getgrist.com/D3257
