Implement batching / multiple scans

[gubpalma]

• For things like PDF conversion or EPUB conversion, it would be nice to be able to process images as a collection / sequence of scans
• The mobile app would also require some changes to facilitate this
  • To be driven from the mobile app, the app would have to signal a "start" / "stop" batch API call sequence
  • On the app this could possibly be an "Capture another page..." button or similar
• The server should assign a unique batch ID to the ScannedImages record(s)
• Workflows would not be triggered until the final "stop" batch command
• Workflows must all be validated to ensure that they can handle multiple files?
  • The ExecutionStepArtifact should possibly be reworked into an array instead of a single object
  • Some steps such as OCR would intuitively process each OCR operation per artifact
  • Other steps such as PDF conversion would (should?) intuitively collate the entire batch into a single PDF (under what title?)