This commit enables the automated sending of daily email notifications to users, who wish to receive one email per day with unread notifications.

Classes:
- GroupController.java: Code improvements
- MediaController.java: Code improvements by adding controllers prefix to routes, implement new notification system for media uploads
- EmailHandler.java: Fix correct subject in sendNotificationsEmail(), implement sendDailyNotificationsEmails() for sending daily notifications via email to recipients wishing to receive only one mail per day
- Group.java: Add constant GROUP_NEW_MEDIA for media uploads, add implementation for media uploads in getRecipients(), improve getTargetUrl() to link to concrete group for appropriate types
- GroupAccount.java: Add deletion of notification per user, if a group account link is removed in delete()
- NewNotification.java: Implement deleteReferencesForAccountId() to delete a reference for a user (required in GroupAccount::delete()), implement findUsersWithDailyEmailNotifications() to get a map of Account instances containing a list of unread/unsent notifications (required in EmailHandler::sendDailyNotificationsEmails())
- NotificationHandler.java: Improve condition in handleMail()
- Global.java: General code improvements, add async Akka process for daily sending of notification emails in onStart()

Templates:
- Emails/notifications*: Improvement in i18n by adding count of new notifications
- NewNotification/group/group_new_media.scala.html: Add template for notification on media upload
- Notification/list.scala.html: Move comment to top to avoid compilation errors as Notification class is deprecated
- Rest: Remove unnecessary condition of notification.isRead as it will never happen, that a read notification is re-rendered

Other:
- messages: Remove unnecessary i18n entries as they are never used