This commit refactors the notification system in the friendship handling.
- FriendshipController.java: Add notification handler, fix code conventions
- PostController.java: Remove deprecated notifications, add notification handler for comments on own news stream
- Friendship.java: Refactor friendship model to implement INotifiable interface
- NewNotification.java: Add method deleteReferences() to delete notifications containing obsolete reference instances
- Post.java: Fix reference deletion from notifications on post deletion, add handling for comments on own news stream
- new_request.scala.html, request_decline.scala.html, request_successful.scala.html: Add templates for friendship notifications
- comment_group.scala.html, comment_profile.scala.html, comment_profile_own.scala.html: Fix comment templates, add comment on own profile template
- menuitem.scala.html: Add div layer to surround notifications
- messages: Add i18n for post comments on own profile and friendship notifications
- htwplus.css: Add styles for notification elements