vague prompts for active steps, feedback to match

- for active solution steps, the step prompts no longer tell you what
you are supposed to do
- however, to compensate, the validation and correctness feedback DO
tell you what you WERE supposed to do
- to make implementing this easier, I restructured the way validation
and correctness tests are performed: instead of being handled within
displayTutorialTrial, it is handled by validation and correctness
checkers passed in by createTrialSpec
- these checkers are also responsible for generating all feedback
content. This seems clearer: all content generation (prompts, validity
feedback, correctness feedback) is in the same area of my code