Enhancement: Do not overwrite worlds/systems/modules on import

If importing data and an existing world/system/module exists with the same name, user should be notified and not allowed to overwrite it without first confirming they want to delete it.
This also means that uploading the same world/module twice would wipe the old content away, which would be needed to prevent having straggling files left over from a previous version.
This would depend on the Feature: Delete worlds feature being implemented first.