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 had a go implementing a custom validator but the specs got brittle and mixing inline model validations seemed to not work. I then tried the each_validator but didn't like the specific magic implementation. I think the `validates :validate_gold_standard` leveraging a poro that know how to validate the schema (possibly versioned later) was cleaner. 

Happy to change it if you want though.