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
I don't think the user's classifications expert status will change much over time but during analysis we could realise that user is an expert and treat all their classifications as expert.

I think the main use case for this was to create the gold standard data set so missing some classifications will happen. @ggdhines @aliburchard thoughts?