TMSS-173: simplified validation of json_strings against schema's. The original design (proof-of-concept) had the validation in the Subtask class, and the suggestion/goal of this ticket TMSS-173 was to change that into a reusable mixin. Using a mixin is certainly possible, and the 1st implementation in this ticket was a good start, but it also showed that the whole class/mixin based approach requires too much administration (keeping track of which fields to use). This new solution with just a validation method is far simpler, and just as reusable.