Unique copies and pages in the database

This MR:

• Adds a read-only Copy._exam_id column. Updates automatically to Copy.submission.exam_id.
• Adds a unique constraint for (Copy.number, Copy._exam_id).
• Adds a unique constraint for (Page.number, Page.copy_id).
• Alters the scans pipeline to catch and handle failed unique constraints gracefully.
• Fixes the raw_scans pipeline to not commit an intermediate step, but flush instead.
• Adds a naming convention for constraints.
• Adds a migration for the changes above.
• Fixes a bug in migrations with newer metadata.

Closes #138 (closed)