Block supporting pages from document collections

See: https://www.pivotaltracker.com/story/show/61885600

• Moved validation of document types in document collections to DocumentCollectionGroupMembership from controller.
• Added supporting pages to list of banned document types for document collections.
• Added a data migration to blat document_collection_group_memberships where document is a supporting page.
• Fixed up the edition factory :with_document trait.