Moved validation of doc type in collections to model

This used to be handled directly by Admin::DocumentCollectionGroupMembershipsController.

Also added some basic validations for document and document_collection_group
to DocumentCollectionGroupMembership.
Document collections don't allow supporting pages

And adds a migration to clean up any which may have been made by accident.
Fixing Edition factory trait :with_document

This commit causes documents built by an edition factory with the 
:with_document trait to correctly set the slug and document type on
the document.

The edition factories in general look like a good candidate for some
serious yak shaving, but this will have to do for now.