General: This commit introduces a new notification concept using pre-rendered notifications (for performance reasons), that are generated asynchronously using Akka. The client retrieves new notifications via WebSockets that reads new notifications asynchronously, too. For the moment, only postings in groups and on profile streams are refactored using this new notification system.

- routes: Add route for WebSocket notification system
- messages, messages.de: Add i18n to the new templates and logging features
- group.scala.html, profile.scala.html: Add templates to pre-render notifications
- NewNotification.java: Implement a new notification model for the new notification system
- NotificationHandler.java: Implement a singleton handler that is able to generate notifications asynchronously using Akka, in future a mail sending feature will be added
- INotifiable.java: Create an interface for all models, that are "notifiable" (like post, friend request, etc.)
- BaseNotifiable.java: Implement an abstract helper class for notifiable models
- BaseModel.java: Replace @MappedSuperclass annotation with @Entity and @Inheritance, this way, the BaseModel is usable as a reference type from other JPA entities
- NewNotificationActor.java: Implement a WebSocket Akka actor to handle the sending of new notifications to the WebSocket client asynchronously
- NewNotificationController.java: Implement a controller that handles a WebSocket channel for new notifications. It is using Akka and the NewNotificationActor to read new notifications from DB asynchronously and send them back to the client in JSON
- Post.java: Inherit from BaseNotifiable and implement INotifiable methods to be able to be used as a new notification, optimize previous code
- PostController.java: Improve code by removing unnecessary or optimizing required parts, integrated NotificationHandler to create
- Application.java: Some code optimizations