Reject Layer2 Address Changes if email notifications enabled but email is invalid

[afk11]

If notifications are enabled but there's no recipient, entering an email lead to a confusing 'AJAX Request failed' error, but when you refresh, the MAC was actually saved.

Edit: following comment from Barry, approached this a little differently

With this PR, if we are configured to email on superuser/customer changes, and the email is not valid, we disallow the change from taking place.

Making these changes highlighted an quirk in the Listener.. If configured to send email regarding superuser changes (disabled for customers), then modifications by a customer would still trigger an email. This isn't documented, and the two separate configuration values suggest they're independent controls.

I had tests covering the listener, but I'd misconfigured the test case and missed it. So I've rewritten the tests to make it clearer how the config interacts with things.

IXP-Manager/tests/Listeners/Layer2Address/ChangedTest.php

Lines 105 to 116 in 6bc919b

	config()->set('ixp_fe.layer2-addresses.email_on_change_dest', 'notifications@ixp.local');
	config()->set('ixp_fe.layer2-addresses.email_on_superuser_change', true);
	config()->set('ixp_fe.layer2-addresses.email_on_customer_change', false);
	$event = new Deleted("42:42:42:42:42:42", $vlanInterface, $user);
	new Changed()->handle($event);
	Mail::assertSent(ChangedMail::class, function (ChangedMail $mail) use ($event) {
	return $mail->hasTo( 'notifications@ixp.local' ) &&
	$mail->event === $event
	;
	});

[barryo]

Nice catch @afk11 - I think in the current implementation, you shouldn't be allowed to change the MAC unless there is some form of notification/handling. Right now, it's email or bust - so perhaps just don't allow it in this case rather than not sending the email.