Large refactoring to allow for the same item to be shared to a group and again with a user in the group with different permissions

Conflicts:
	lib/group.php
	lib/group/backend.php
	lib/group/database.php
	lib/group/interface.php
	lib/public/user.php
	lib/user.php
	lib/user/backend.php
	lib/user/database.php
	lib/user/interface.php