It was necessary to deprecate the filter
mailpoet_archive_email_subject and create a new one since the type
of one of the parameters passed to the filter had to change.
[MAILPOET-4307]
- Based on how the NewsletterShortcodes service is implemented it's safer to instanciate
it inside the user method and not inject it in the class contructor
- Revert back the changes added to MailPoet\Newsletter\Shortcodes\Categories\Subscriber::process
[MAILPOET-4069]
