Secure EMail Template Editing with Preview and enhanced UI

[tobias74]

Cuurently we have a very rudimentary EMail-Template Editor in place. There is no preview functionality. Templates (which contain TWIG-Template-Code) are saved, without version management or history. And they are live immediatly.

We should implement a thorough editing process, including some sort of history and revert functionality, in case something goes wrong.

Sending confirmation emails is one of the central functionalities of this software, since the confirmation mail is a key part of the customer process. Failures in this part of the software will basically stop the main functionalities.