Show correct answer and feedback when overriding grade (backport of MDL-74495)

IMHO MDL-74495 was borderline bugfix/improvement: the fact that one could not see what answers are correct (e.g. in a cloze question) made it unnecessarily cumbersome to manually override grades. OTOH, one could argue that the issue added a new functionality that was simply not there before, it did not fix an error.

I would like to backport MDL-74495 mainly for the following three reasons:

• it greatly simplifies the process of manually overriding a grade, especially in cloze questions
• the fix is very simple and short, basically just one single line of code plus a behat test
• it does – IMHO – not have any negative side effects, even to people who did not miss or do not need that functionality