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
Re missing the classifications, I should have said missing flagging classification as gold standard. Say the user is marked expert half way through a project as a project attracts expert classifiers after launch. 

If we don't want to enable this workflow then we'll add an explicit expert role for 'expert' classifier that cannot be changed after project launch. Seems a little arbitrary to me though.