Make diff and judge message output more consistent between default and custom validators

[eldering]

The judging_run_output.output_diff column gets filled with the contents from the feedback/judgemessage.txt file (optionally) generated by the output validator, per specification. It is not the actual diff of team output against jury answer, which is calculated in the runDiff function.

We should see if we can make the things that we store and display for judgings diffs and similar feedback more consistent. Can we maybe even merge these? (Likely not, since we'd then have to give up on the nice diff HTML formatting.)

Slightly relevant: #2256