Deleting document deletes collection memberships

www.agileplannerapp.com/boards/173808/cards/8071

a document collection cannot be re-editioned if its
groups contain deleted documents. if we destroy all
document collection group memberships when a document
is destroyed, this issue will not occur, and will
also make sure references to documents are valid.

also includes a data migration to delete ~230 dangling
`DocumentCollectionGroupMembership` records referring
to deleted documents, which were created in absence
of `dependent: delete_all`.
Merge pull request #1825 from alphagov/carry-forward-collection-documents-to-new-editions

Deleting document deletes collection memberships