When we create a new email in the new editor we ensure there is
also an associated NewsletterEntity.
This commit improves this functionality by replacing custom code with
NewsletterSaveController, which ensures that the sender name and address are set.
[MAILPOET-5646]
codeception/verify 2.1 removed support for expect()->stringContainsString() so we need
to replace it with verify()->stringContainsString().
[MAILPOET-5664]
This changes how the editor is initialized. The integration initialization class
doesn't get the engine class anymore, but integration is initialized via
mailpoet_email_editor_initialized action triggered from the engine.
This allows multiple integrations to hook into the engine.
[MAILPOET-5540]
