Revert "Do not treat earnings type as asset."

This reverts commit 18ec42f33983476722a5c64a138e1bd27e86a90e.
Revert "Costs accounts are no asset accounts, but earnings is."

This reverts commit 0abf43e71c0d7490ec0e3ff364247b1bba78ddad.
Fix Booking.balance_by to not include non-related bookings.
Implement .balances and .accounts scopes on Booking.