[5.x] Allow conditional sending of form submission emails

[stephensamra]

Overview
Introduces a new FormSendingEmails event that is dispatched prior to SendEmails (the job that sends emails when new form submissions are received).

If a listener of FormSendingEmails returns false, the SendEmails event will not be dispatched (similar to how FormSubmitted works).

This enables the conditional sending of form submission emails.

Motivation
I have a use case where I'd like to examine a form submission and only send the email(s) based on certain criteria.

I think a good example for this would be spam detection: if a submission is flagged as spam, perhaps we don't want to send an email about it. However, we still want the submission to be saved so that it can be manually reviewed at a later date.

Example listener implementation:

<?php

namespace App\Listeners;

use Statamic\Events\FormSendingEmails;

class HandleFormSendingEmails
{
    public function handle(FormSendingEmails $event)
    {
        if (SpamDetector::check($event->submission)) {
            return false; // No emails will be sent if we reach here.
        }
    }
}

[edalzell]

You can already set the job used to send emails in the config, can you not use that?

Also, the spam case is already handled

[stephensamra]

Thanks @edalzell, I must have an old config file that doesn't have that option. I should be able to use that.