#49 This commit summarizes all new notifications for a specific post per user.

Classes:
- INotifiable: Instead of just instantiating a new notification in the NotificationService, the INotifiable realizing class decides, how to get a new or already given notification instance
- BaseNotifiable: The default case is just returning a new notification
- NotificationService: Make use of the INotifiable::getNotification() method and create/update the notification depending on a given or not given ID
- Notification: Update timestamp correctly, if notification is modified, use updatedAt in findByAccountId(), add method findByReferenceIdAndRecipientId()
- Post: Return parent instance as reference for notification, if given, otherwise this

Templates:
- Notification/post/profile.scala.html: Add truncated content to notification text for posts on own profile
- Notification/view.scala.html: Improve notification show all template

CSS:
- htwplus.css: Add class "hp-bold" for font-weight bold

Other:
- messages: Improve i18n for notification show all template
#47 This commit shows a message that a notification is not existent anymore, if already deleted.

Classes:
- NotificationController: Show message

Other:
- messages: i18n for message
Merge pull request #54 from socia-platform/48_leaveGroupConfirm

#48 leave group/course confirm modal
Merge pull request #55 from socia-platform/42_postContentLength

42 post content length
Merge pull request #56 from socia-platform/47_notificationRemove

47 notification remove
Merge branch 'master' into 49_summarizeComments

Conflicts:
	app/models/Post.java
	conf/messages
Merge pull request #57 from socia-platform/49_summarizeComments

#49 This commit summarizes all new notifications for a specific post per...
#48 refactor headline
Merge pull request #58 from socia-platform/48_leaveGroup

#48 refactor headline
#52 refactor group/course invitation wording
Merge branch '52_inviteCourseBug' into develop