fix(notifications): only add project subscription as task subscription when the user is not already subscribed to the task

Before this fix, a project subscription object was added twice to the list of subscriptions for a task when the user did not subscribe to the task directly. This caused the user to receive a comment notification twice for a given task.
This was probably a regression from efde364224.

Resolves https://community.vikunja.io/t/e-mail-notification-twice/2740/18

(cherry picked from commit 2c9becec101c14dd744444fffe244510e8394323)

pkg/models/subscription.go

@@ -259,7 +259,7 @@ func GetSubscriptions(s *xorm.Session, entityType SubscriptionEntityType, entity
 				delete(subs, task.ID)
 			}
 
-			if !hasTaskSub {
+			if !hasTaskSub && !hasProjectSub {
 				subs[task.ID] = psub
 			}
 		}