Removed duplicate pairs from the table of student comparisons. In the process changed how the table is rendered

Description

What?

• Changed how the table in the exercise view is rendered. Duplicate pairs are removed.
• Replaced outdated unit tests in matcher/tests.py and data/tests.py to test new changes

Why?

• An improvement suggested in issue 31.
• Improves overall usability by not cluttering the view with the same comparisons multiple times

How?

• Changed how the function top_comparisons in models.py works, to return the list of comparisons in a set instead of a dictionary of lists, thus removing the duplicates. Additionally rewrote the django template for the table to support this new data structure.

Fixes #31 (partly)

Testing

Remember to add or update unit tests for new features and changes.

• How to test your changes in A-plus
• How to test accessibility

What type of test did you run?

• Accessibility test using the WAVE extension.
• Django unit tests.
• Selenium tests.
• Other test. (Add a description below)
• Manual testing.

Tested with 5 submissions and 100 submissions locally. Submissions were "fake" and essentially identical. Testing with real data would be preferred before merging.

Did you test the changes in

• Chrome
• Firefox
• This pull request cannot be tested in the browser.

Think of what is affected by these changes and could become broken

Translation

• Did you modify or add new strings in the user interface? (Read about how to create translation)

Programming style

• Did you follow our style guides?
• Did you use Python type hinting in all functions that you added or edited? (type hints for function parameters and return values)

Have you updated the README or other relevant documentation?

• documents inside the doc directory.
• README.md.
• Aplus Manual.
• Other documentation (mention below which documentation).

Is it Done?

• Reviewer has finished the code review
• After the review, the developer has made changes accordingly
• Customer/Teacher has accepted the implementation of the feature

Clean up your git commit history before submitting the pull request!