Rework attachments drectory/ies settings

[emanuele45]

The first and main proposal is to drop the support for the single directory option and have a default multi one.
I would go with the year/month pair, that should be enough in the vast majority of the cases.

The second is drop the serialized structure in favor of the simpler json.

Also the code requires some reorganization, but I guess this will come while working.

[Spuds]

😸

[Vekseid]

My suggestion would be to have hexadecimal hash subdirectories, rather than by date.

json rules phpserialize drools.