Avoid cheeky and creative storage

[a-surfer]

We are currently accounting disk usage based only on content length of WebDAV members. We are not taking into consideration the storage space that properties from resources (that is members and collections) take.

When we start enforcing quotas and so, creative users could start storing all their file contents in properties from resources instead of the "bodies" of resources themselves, effectively bypassing their quotas and the limitations we want to establish in the system.