Expose problemoutput & checkoutput with `/runs`

[vmcj]

I know the analysts prefer to get such output for analysis and now need to rerun submissions to get this output.
I also know that exposing all this data (or even storing it) might have a huge overhead so I propose to truncate this to a certain number and either only give the beginning or the end (or just remove the middle).

I don't think this is relevant for the current spec but @nickygerritsen suggested to put it on the agenda.