Add ticket thread class.
Fully implement thread entry class and types (Note, Message & Response)
Add method to sanitize rich/html text. Addd html encode/decode methods. Remove html encode on display (encoded text saved in db)
Refactor ticket related methods/utils to use thread model.
Balance html tags & neutralize unsafe tags on emailed html'ed emails.
Decode html entities - outgoing emails are plain text for now
Don't show reply/response section on new ticket form if the staff can NOT post replies... also checked in Ticket::open().
Refactor the class to use data hashtable instead of individual vars in getters.
Fix minor spacing/formatting issues.