ADD Show diff between the expected output and the actual output in the autograder

[PIG208]

The plan is to add a column called diff to SubmissionTestResult, which is usually None.

I'm adding a column instead of a special directive that can be parsed when it shows up in stdout. Because we will have to implement and maintain an extra parser in the frontend, while having a separate field allows us to feed it to react-diff-view directly.

Fixes #254

[wabscale]

Hey you should hold off on this. This is going to be a piece of much larger changes to the way that autograding happens. We're going to be doing this during winter break.

[PIG208]

I will do a quick push tmr to kind of see how this turns out.

[PIG208]

OK, it seems to be a bit more complicated than I expected. Because now I'm trying to set up test cases before actually making the changes. So it might take longer.

[PIG208]

[PIG208]

I just updated the PR. Specifically, I reworked test_lines in bf3f1e5. Previously we just preprocess all the lines and compute the diff of the entire outputs. This improves the performance by only parsing the diff within a limited context where we have the first mismatch.

[wabscale]

I'm going to get on later and push some changes before rebasing this. I need to adjust a few things so that it will fit into the changes to the autograde tests format.