Global Mail Template Support

[JUVOJustin]

Allow adding a global mail template. The goal is not to allow frontend editing of a mail template, but to add static HTML or twig code that will enclose the message.

Best would be to use a variable to pass to the template, and it should be filled by the "message" in the wp_mail hook.

• Add section in optionspage to add twig with content block reference
• Add checkbox to disable global templating
• Live rendered preview by ajax