- GroupController.java: Implement new notification system for group invitations
- BaseNotifiable.java: Add temporaryRecipients and temporarySender for assigning not stored senders/recipients. Implement method addTemporaryRecipient().
- Friendship.java: Fix order of "public static final"
- Group.java: Refactor Group to be a BaseNotifiable implementing the INotifiable interface.
- Add group notification templates in views/NewNotification/group and i18n in messages