Move data-related methods to DocumentData

…and shim them in Document
Make sure to delete DocumentData

…of last referencing document on Document deletion
Update schema for DocumentData

• Make sure changing DocumentData objects is never denied directly
• Deleting a DocumentData while still being referenced should be denied
Merge branch 'document-deduplication' of github.com:rapila/cms-base into document-deduplication

* 'document-deduplication' of github.com:rapila/cms-base:
  Fix memory limit causing migration to abort