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]
I envision the StylesController as the source of information about
styles settings in both the editor and renderer.
We will add email styles settings and these will be accessed in PHP via this service.
[MAILPOET-5540]
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]
