* [CHD-3689] [Virtual Attendants/Placeholders/Mail] Virtual Attendants and snippets can now reference message header values in the 'message_headers' placeholder (for instance 'ticket_initial_message_headers').  The result is an associative array with the header name in lowercase as the key, and the corresponding value.
* [Virtual Attendants/Snippets] The 'Insert' menu for message placeholders now includes 'message_headers' as an option.
* [Virtual Attendants/Mail/Headers] Message and Ticket behaviors can now create conditions based on the headers of any linked message record (initial, initial response, latest, etc).  Previously only a few of these were available.  The old 'header' conditions will be automatically converted to the new 'headers' options.  Performance has also been improved for multiple header operations, since all the headers are loaded once for the first placeholder, where previously they were loaded every use (even if duplicated).
[Code Cleanup]
6.6.4-dev checkpoint