Tidy up EmailInterface body functions

Problem/Motivation

The concept of appending to the body is left over from an early design idea before it was possible to set the body in the Mailer Policy GUI.

1. It doesn't seem so useful now. Every use of appendBodyEntity() within this module really intends to do setBodyEntity().
2. It doesn't work reliably if you mix setBody() followed by appendBody(). If the first body uses a theme that doesn't render children then the appended data will be lost.

Proposed resolution

1. Change appendBodyEntity() to setBodyEntity()
2. Remove addBody()

If there is sufficient demand we could add an $append argument to setBody() and fix the problem with mixing set/append.

Remaining tasks

User interface changes

API changes

See above.

Data model changes