Relocates some ticket responsibilities to a TicketDecorator.

There were a couple of behaviours that were bothering me regarding
ticket creation and updating: the setting of the default subscriber in
an after_create callback in the Ticket model, and the sending of Ticket
update notifications from the TicketsController. I've pushed both of those
into the new TicketDecorator. This allows the Ticket model and TicketsController
to more closely focus on their core responsibilities.