Removes TicketConductors.

The recent ticket changes removed the need for the ticket conductors.
The simple tasks they were left to perform after the introduction of
the state_machine weren't enough to justify their presence.
Adds async mailing.

Sidekiq has been added to the mix in order to ensure the Ticket mailing
tasks don't hold up requests on their way through the relevant
controllers. Also revises the TicketNotifier service to be the single
entity responsible for performing all Ticket related mailing. This removed
some overlooked duplication in the CommentConductors.

The CommentConductors are now ripe for further refactoring themselves; they
are currently structured in a manner that clearly calls for a simple
inheritance structure that would eliminate further redundancy between them.