Scanned exams - decouple student matching from scanning

[donny-wong]

MarkUs has automatic student matching when scanning exams. This happens during the scanning of exams. For example, you can scan by student number or UtorID, and we annotate where it should look for this:

however, if one of the identifier fails (e.g. student number), we would like to try matching on the other one (e.g. UtorID). For this to happen we will need to do another scan but this time annotate the UtorID section.

To improve this by not needing to scan again, we need to decouple the student matching component from the scanning. So then we can do rematching without needing to scan again.