[Feature Request] Button to diff student's work

Currently, in the assignment overview, there are two buttons: View Test and Go to repo. It will be nice to have an additional button that shows the diff between the master branch and the template repo.

The diff functionality is already implemented in GitHub through the compare commits feature. However, for that to work you need to know the hash of the last commit of the classroom setup script.

Having this link automatically generated by the GitHub classroom would simplify the grading steps that cannot be automated such as code readability or text answer.