#42 large text
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
#48 leave group/course confirm modal
#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