add nestable page collection
merge in properties from default pages
change approach: embed default pages in collection/homepage views
include parent field when editing a page
make it possible to clear the parent property (and linkTo properties in general)
validate that page parent is not a collection default page (and add registry of validator functions)
show validation errors next to their respective fields
make Item.__parent__ a property to avoid the need for overriding it with a no-op on Page
Merge pull request #356 from ENCODE-DCC/1656-new-page-collection

1656 new page collection