* [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]
Merged '6.6.4' into '6.7'
* [CHD-3172] [Virtual Attendants/Mail] The 'initial message' and 'latest message' placeholders are now available in all message-based Virtual Attendant behaviors.
* [Virtual Attendants/Simulator/Usability] The Virtual Attendant simulator now only makes conditions editable if they are used by the behavior being tested. For instance, an auto-reply behavior that uses the conditions 'message is new' and 'message is outgoing' will now only show those two values in the simulator, which drastically improves usability. Previously, the values of every possible condition were shown (potentially hundreds) even though most of them weren't useful for testing the current behavior.
6.6.4-dev checkpoint
6.7-dev checkpoint