MDL-37178 quiz attempts: make it easy to add a Previous button.

The quiz intentionally only has a Next button at the bottom of the
page, so it is clear what students should do next, but some people
insist they want a Previous button too. The only way to add one
involves changing core code, so to make it easy for them, we
put those changes in, and indeed generate the Previous button and
just style it display: none.