1.1.3dev: Adopt notification subscriptions (see #4056)
* DB upgrade to add new table `notify_subscriptions`
* new `INotificationSubscriber` interface
* new ticket subscriber implementations replacing existing logic:
  * `TicketOwnerSubscriber` implements `always_notify_owner` option logic.
  * `TicketUpdaterSubscriber` implements `always_notify_updater` (current updater) option logic.
  * `TicketPreviousUpdatersSubscriber` implements `always_notify_updater` (previous updaters) option logic.
  * `TicketReporterSubscriber` implements `always_notify_reporter` option logic.
  * `CarbonCopySubscriber` implements CC ticket field logic.
* new ''Notification'' preferences panel for configuring subscriptions.

Based on Announcer plugin.

git-svn-id: http://trac.edgewall.org/intertrac/log:/trunk@13469 af82e41b-90c4-0310-8c96-b1721e28e2e2