Use ckEditor for editing email body

Problem/Motivation

Currently the "Email HTML" text format uses a plain text editor. It would be much better to use ckEditor instead.

Proposed resolution

The main reason we don't yet use ckEditor is that it corrupts TWIG control structures. There are some ideas how to deal with that here: https://stackoverflow.com/questions/20977910/editing-twig-templates-in-c...

Remaining tasks

User interface changes

API changes

Data model changes