Add the disabled_file_integrity_check option to disable downloaded files integrity check

With the new blob store key strategy, managed, digests sent by the server are no more MD5 ones.
It breaks Direct Edit, for instance, as after having downloaded the file, an integrity check will be done. In that case, it will never make it and the Direct Edit will never be possible.

We should introduce a new option to control that check: disabled_file_integrity_check defaults to False.

It will be tweakable locally and from the server.