fix(subscriptions): cleanup and simplify fetching subscribers for tasks and projects logic

Vikunja now uses one recursive CTE and a few optimizations to fetch all subscribers for a task or project. This makes the relevant code easier to maintain and more performant.

(cherry picked from commit 4ff8815fe1bfe72e02c10f6a6877c93a630f36a4)

pkg/models/tasks.go

@@ -1573,10 +1573,11 @@ func (t *Task) ReadOne(s *xorm.Session, a web.Auth) (err error) {
 
 	*t = *taskMap[t.ID]
 
-	t.Subscription, err = GetSubscription(s, SubscriptionEntityTask, t.ID, a)
+	subs, err := GetSubscriptionForUser(s, SubscriptionEntityTask, t.ID, a)
 	if err != nil && IsErrProjectDoesNotExist(err) {
 		return nil
 	}
+	t.Subscription = &subs.Subscription
 
 	return
 }