Add kill-switch

Problem/Motivation

There are scenarios, where events should still be recognized and notifications being created, but their delivery should be prevented, e.g. in development or other non-live stages.

Proposed resolution

Such a kill-switch should either be done with a setting or as a flag in Drupal's state environment. When it's being enabled, all created notifications should get their flag "delivered" being set to 1.

Extra attention should be spent to recipient selection plugins that implement immediate delivery.