`Plagiarism checks`: Fix plagiarism split view for exam exercises

[Strohgelaender]

Checklist

General

• [ ] I tested all changes and their related features with all corresponding user types on a test server.
• [ ] This is a small issue that I tested locally and was confirmed by another developer on a test server.
• [x] I chose a title conforming to the naming conventions for pull requests.

Client

• [x] I followed the coding and design guidelines and ensured that the layout is responsive.
• [ ] I added multiple integration tests (Jest) related to the features (with a high test coverage), while following the test guidelines.
• [x] I added multiple screenshots/screencasts of my UI changes.

Motivation and Context

Currently, it's not possible to access the plagiarism split view for exam exercises. A Bad Request Error is shown becuase the request contains undefined as courseId.

Description

For exam exercises, the course has to be retrieved using the exercise group. This PR solves this by calling a helper method.

Note: Artemis does not support the whole plagiarism process for exam exercises right now. This PR only fixes viewing the findings, more changes have to be done in a followup.

Steps for Testing

Prerequisites:

• 1 Instructor
• 1 Exam programming exercise

1. Run Plagiarism checks for the exercise
2. Check that the split view works.
3. Make sure that the Accept / Deny buttons are disabled.

Code Review

• [ ] Review 1
• [ ] Review 2

Manual Tests

• [ ] Test 1
• [ ] Test 2