THe Main Subsite has a "magic" content folder folder_id -100.
Additional subsites that are mounted in the site-map do not get a
content-folder. Should they be assigned one by default? Or should a
package that needs the folder check, and if it is not created, create one?
Edit-This-Page allows you to administer content from the site root
using this content folder. To admin a subsite you need to create this
folder. I just don't know if we should automatically create one when
we add a subsite package instance or not.