add gold standard flag to classification

If the user is an 'expert' on the project then add a gold standard flag to the classification, closes #237
@edpaget I'm not sure actually - thought I was and thought about it - for example, we could have a lion expert helping classify photos in Snapshot Serengeti who isn't so good with zebras. Seems a little far fetched but not impossible. In such a case we would need a way to give that expert only pictures of lions or have that expert only say "yes lion" or "no lion". BTW we also need a way for research groups to upload their own gold standard data which they created outside of the interface - some possible issues with choosing images at random. With Snapshot Serengeti they took a bunch of power users and manually combined their classifications. So we would need a way to allow for those combined classifications to be uploaded. Although this would mean that the gold standard data is being created after the project launch which doesn't seem good. Basically Ed - I'm not sure and I think we need to think a bit more about the role of experts. @aliburchard - thoughts?