Atomic Operations for Document-Wide Deletions

[rsimon]

Implement atomic operations for 'Delete Document' and 'Delete Document Annotations'. Both of these are tricky since a lot gets deleted:

• Annotations (batch delete)
• Annotation versions (batch delete)
• Contribution records (batch delete)

...and in case of 'Delete Document' additionally...

• Document metadata & metadata of each filepart (these should be handled atomically by the DB)
• Document file uploads