Clean up problem editing flow

[cayhorstmann]

This from Maurice Naftalin: The workflow for refining a CodeCheck problem is a bit laborious: because re-submitting your input takes you away from the submission page, you have to either repeatedly go back to the submission page, or click on the link on the response page to go to the edit URL. I did the second of these, and ended up with a confusing multiplicity of open tabs. It would be nicer if the the result of submission was to stay on the same page (with the zip upload field unaltered and perhaps with the unit test results appended on the same page), so that after changing the zip file, you could just click on the upload button again and then just refresh the student tab.