CVR manifest import

[ranweiler]

See: #16.

• [x] Include ballot image source path in Ballot type
• [x] Include ballot image source in JSON responses
• [x] Add ballot manifest upload button
• [x] Add ballot manifest view button
• [x] Disable view button unless uploaded
• [x] Disable upload button if already uploaded
• [x] Implement submitBallotManifest action creator
• [x] Ensure we're fetching existing ballots from backend
• [x] Disable ballot manifest view button if no ballots have been uploaded
• [x] Add vendor processing code for handling ballot manifest upload
  • [x] Check manifest type for "ballot" value
  • [x] Parse manifest JSON to extract image file locations
  • [x] Add prepared statement to save manifest-derived ballot data
  • [x] Save src_path's when processing manifest JSON, use empty file_path for now
• [x] Add integration test for ballot manifest upload
• [x] Rename ballot_image table to ballot
• [x] Enable "Upload" button
• [x] Enable "View" button