FIX: Respect personal_email_time_window_seconds in group SMTP (PR #13630)

For other private messages we have the site setting personal_email_time_window_seconds (default 20s) which allows people to edit their post etc. before the email is sent.

This PR makes the Jobs::GroupSmtpEmail enqueuer in the PostAlerter use the same delay.


I feel that this spec will be flaky. Maybe we should freeze time?