Nice to see last push time in gh classroom assignments

I would like to be able to see not just 'commit count', but also last commit time, so that my graders can see when a student has submitted 1 day late, 2 days late, etc.

Start at the gh classroom assignments output where commit count is shown, and inspect how commit data is retrieved and displayed. Done means graders can see each student's last commit time and use it to determine whether the assignment was submitted late.

Written by the indexing model from the issue text.