Allow personalised screener questions#298

Right now it’s not possible to add specific questions to the screener, only to select among a list of pre-determined criteria. This is extremely limiting, and makes recruiting for the right participants impossible. The workaround I was presented with - to create two tests, one of which is a screener questionnaire - it’s not only convoluted, but also expensive as it means paying for two tests, when in fact it’s the same study.