Fixes bug introduced by bd25627198
What happens is we send notifications to everyone involved in the group inbox topic about new posts, however we pass the param `skip_send_email_to: email_addresses`. In the above commit I removed the group email address from this `email_addresses` array. This breaks the IMAP inbox because we email the group with the reply, and the IMAP sync tool finds this email and opens a new unrelated topic with it.