Draft: "Detach image from page"

Closes #481

Changes:

• Adds Image table
• Migrates page path to image path (without moving the images?)
• Removes global scan status and message (now they belong to each individual image)
• Image is saved on a global path shared across all exams with unique uuid
• Scan view now shows a count of processed/failed/waiting images.

What this MR is not (but will be done after this is merged):

• Split processing
• Update UI to show status for each image (update MissingCopies)