Add support for creating hub subpages to SpecialEditCollaborationHub.
When not called on a particular page, it should ask for a page title and try to edit that. When called on a page that doesn't exist, it should display fields for creating the page, and then create the page (setting the content model) upon submit.
This requires different rights than just editing, so we need to also check if the user can create pages/change content models before doing any of this.