Problem comments feature

[RodionGork]

After problem (especially quiz) is solved, it could be good to show some comments on the result page, just below the success message.

I propose to add it similarly to how problem statement or wiki page is displayed: check the blobs
table for t.<problem-id>.a.en key, and if it exists, display it parsing the markdown. Check explanations in #48 about how to add blob.