Start over button should hide hints and solutions as well

[mine-cetinkaya-rundel]

Currently, the "Start over" button only seems to affect the code in the webr cell. I also expect it to clear away any hints/solutions I may have revealed, and I think others would as well. I think it would be a good enhancement for the "Start over" button to return to "factory settings" for everything associated with an exercise, not just the code cell.