ADD a button on the results page that shows the git difference

[YinWang3026]

Is your feature request related to a problem? Please describe.
As Grader, I would like to quickly see the changes that students submitted against the base commit.

Describe the solution you'd like
Add a button on the results page that links to a GitHub page that shows the difference between HEAD commit and the base commit.
Aka a link to the Github URL to the repo with "/compare/head_commit..base_commit" appended.
https://docs.github.com/en/pull-requests/committing-changes-to-your-project/viewing-and-comparing-commits/comparing-commits#comparing-commits

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

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