Add task assignment notification system

- Introduced `AssignTaskEvent`, `TaskAssignEventSubscriber`, and `AssignTaskNotificationFlagProvider` for task assignment notifications
- WIP : Added notification templates for task assignment title and content
- Updated `SingleTaskController` to dispatch `AssignTaskEvent`
- Adjusted translations and service configurations accordingly

src/Bundle/ChillTaskBundle/Controller/SingleTaskController.php

@@ -23,6 +23,7 @@ use Chill\PersonBundle\Entity\AccompanyingPeriod;
 use Chill\PersonBundle\Entity\Person;
 use Chill\PersonBundle\Privacy\PrivacyEvent;
 use Chill\TaskBundle\Entity\SingleTask;
+use Chill\TaskBundle\Event\AssignTaskEvent;
 use Chill\TaskBundle\Event\TaskEvent;
 use Chill\TaskBundle\Event\UI\UIEvent;
 use Chill\TaskBundle\Form\SingleTaskType;
@@ -525,6 +526,13 @@ final class SingleTaskController extends AbstractController
 
                 $this->eventDispatcher->dispatch(new TaskEvent($task), TaskEvent::PERSIST);
 
+                if (null !== $task->getAssignee()) {
+                    $this->eventDispatcher->dispatch(
+                        new AssignTaskEvent($task, $user),
+                        AssignTaskEvent::PERSIST
+                    );
+                }
+
                 $em->flush();
 
                 $this->addFlash('success', $this->translator->trans('The task is created'));