Emails: add Customizer integration.
This integration allows appearance changes to be easily made to emails, and previewed. For example, font size, background and font colors, and more.
A new submenu has been added under wp-admin's "Appearance" menu that takes the current priviledged user directly to the Customizer preview for a randomly-selected email.
See #6592. Props timersys, mercime, boonebgorges, hnla, DJPaul.