[Bug 1091781] Add helpful votes count to Questions API.

I bet this is going to have merge conflicts with the other Questions API PR I made. Oh well. I'll make sure to merge things right this time.

r?
So... I'm pretty sure these are "I have this problem too" votes. Is that what you want? When I hear Helpful votes, I think of votes on answers. Not sure what they want for real, but I wouldn't call this field helpful votes.