refs https://github.com/TryGhost/Product/issues/3651
- This is a security fix that addresses an issue causing malicious users
to abuse the test / preview email API endpoint.
- We have multiple procedures in place now to limit such users.
- First, we now only allow one email address to be passed into the
`sendTestEmail` method. This method only have one purpose, which is to
compliment the test email functionality within the Editor in Admin and
therefore have no reason to send to more than one email address at a
time.
- We then add an additional rate limiter to prevent a user from making
multiple requests, eg via a script.
- The new imposed limit is 10 test emails per hour.
