Better user interface for job progress

[knrafto]

We shouldn't be limited to a text log - each job should have a custom way to show its progress or eventual result. For example, our autograding job can display a progress bar and a histogram of scores received, and our GitHub search job can show links to any repos it found or issues it created.

We could do this by storing some JSON data for each job, and the server re-renders a template based on that data periodically.