Allow prolific users to sign up with only their prolific ID/email without verification about congame HOT 3 CLOSED

Change the scope of this to: streamline simple study participation where we don't really care about security that much.

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Design: create a new user [email protected] for each study that does not require login and verification, but creates a new account for each new user cross session-id. @MarcKaufmann : Check + in the email works.

Potential problem: getting spammed to create new users on the DB. Allow this feature only for active studies that are meant to be used in this way. (Deal with this probably later.) Or do rate-limiting per IP.

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This will not work, since prolific does not allow emails with + through. So we continue to require sign up.

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