Allow container pages that don't generate a URL segment

Some functionality cascades down the page tree, like approvals or permissions. Sometimes you want to group pages together so you can apply values and functionality to branches of pages, but these necessarily affects the URL, since any page generates a new URL segment.

"Container" pages can't be accessed directly, but they still generate a URL segment, so they can't be transparently used for grouping content.

It would be helpful to allow a type of container page that doesn't generate a URL segment. When ancestors are collected to form a URL for a descendant page, this type of page could simply be skipped and its segment omitted.