Create new Model FeedbackLine and integrate it

Description / Overview

The FeedbackLine-Model has to be added, so line-by-line Feedback is allowed by multiple tutors

Use cases

The first usecase is, if two tutors, who are feedbacking the same submission have a conflict in one line, but not the whole submission. If this happens, a line-by-line feedback is better, so as the conflict is quicker to detect.

This structure allows us generally to have multiple feedback's, without having multiple feedback-objects, but multiple feedbackline-objects.

Feature checklist

• new Database-model "FeedbackLine"
• easy-to-use-system for resolving conflicts

/label Feature proposal