Add Twig support to email body

Problem

Generated emails should support conditional logic, advanced formatting, and embedded styles.

Solution

Allow users to use Twig to generate HTML and plain text email message.

Tasks

• Add 'twig' boolean to email handler settings
• Update email handler scheme
• Update email handler configuration form
• Update generated email to use twig.
• Update scheduled email handler.
• Add permission to edit Twig template.
• Possibly update twig help functions to support advanced formatting.
• Write tests