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.
Merge pull request #1140 from alphagov/block-supporting-pages-from-document-collections

Block supporting pages from document collections

https://www.pivotaltracker.com/story/show/61885600