[TASK] Add SignalSlot for successful opt-in confirmation

Adds a SignalSlot "AfterSuccessfulConfirmation" to
FormsController::optinConfirmAction()

It is triggered after a successful hash validation.

Resolves: #59395
Releases: 2.0, 2.1
Change-Id: I7ccc77c77ba14ad2bd27d97eac3102bc856fb31a