Improvements to Diff Viewer

[20wildmanj]

Request from 122 (Iliano):

the submission diff viewer has been a significant time saver when investigating possible AIVs, not to speak about the improved accuracy. A couple suggestions:

• Could you mark clearly which side is which version? Right now it's not obvious and once I figure it out I forget the next time I need to use it.
• An obvious extension would be to give us a way to compare the current student/file/version with the same file of an arbitrary student and version. You could for example keep the default behavior the same, but provide a dropdown menus for student and version.