Add number of downloads as new limit option for Access Grants

That´s great! I haven´t thought about this until now. I´ve always thought about the limit by number as a security measure and I really like it! But now, as a cost control, I like it even more. The only disadvantage I see is the “amount of time” it could required if someone doesn´t see that the limit number of downloads has been reached (until they set a new limit it could pass minutes or hours so sometimes this could make things more difficult and slower)