This commit creates a notification for each new media and removes them, if media is deleted.

Classes:
- MediaController: Create notification for each media
- Group: Clean up deprecated code from media notification
- Media: Implement INotifiable interface

Other:
- media_new_media.scala.html: Move template from group/group_new_media.scala.html and add file name of media into template
- messages: Change i18n for media notification
#50 When clicking on the link of a media notification, the user is forwarded to the media page of the group
fix data exception
Merge branch '47_notificationRemove' into develop
#48 leave group/course confirm modal
Merge branch '48_leaveGroupConfirm' into develop