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.
<!-- NOTE: All pull requests should have tests (rspec in Ruby, qunit in JavaScript). If your code does not include test coverage, please include an explanation of why it was omitted. -->