REFACTOR: Logic changed on how we add the user to a list.

[andrewlimaza]

• Updated logic on how we add a subscriber to a MailPoet list to prevent fatal errors in some cases. This fixed an issue with guest checkouts.

[andrewlimaza]

This fixes a fatal error where it's unable to send the subscribed confirmation email, when trying to add the user to a list.

[dparker1005]

After looking into this a bit, I have concerns about using the addSubscriber() method as I think we may hit the "This subscriber already exists" exception and the user may not actually be added to the MailPoet list:
https://github.com/mailpoet/mailpoet/blob/d4e239ace521341ff9015d15cb16c5e93bbf9ff9/mailpoet/lib/API/MP/v1/Subscribers.php#L117-L123

Looking into the original fatal error again, it looks like this fatal error shows when MailPoet is trying to sent emails via their premium email service instead of just letting the website send the emails. I was able to disable connecting to the premium email service by going to MailPoet > Settings > Send With... and selecting "Other".

Let me know if this makes sense or if you still think we need this PR