Add ability to trip tags on-request
Cleanup and make HTML safe by default
Strip empty lines in TEXT content
Decide the thread body type at email parsing time.

This would in theory allow us to decide at the email level the content type
to accept.
Fix fixup routines for JSON/XML parsers to use the new ticket thread body
model.
Remove quoted response removal - it's now handled at the email parsing
level.
Handle cases where thready body is sent in as a string

We're going to assume HTML of html thread is enabled - otherwise text is
assumed.
Assume system generated internal notes are HTML when not specified