Refine exercise completion logic when allow_skip = FALSE

[gadenbuie]

Addresses #616 by augmenting the exercise_submission event to include a completed item. This boolean value indicates whether or not the submission should count toward the completion of the exercise and therefore the current section. This is primarily relevant when allow_skip: false is set in the tutorial YAML heading.

Previously, as described in #616, any exercise submission counted toward completion, including pressing "Run Code" on an empty exercise input. This will no longer be the case.

The new logic is set in the exercise_result event handler, which is where the exercise_submission event is emitted. There, we use the exercise cache to access the full exercise object and augment the exercise_submission event data with a new completed item. The full logic is to:

1. Only set correct when submission doesn't require a check
2. Add completed to the data. The flag is TRUE when there is submission code and
   • the exercise doesn't have any grading
   • OR the exercise has grading code (either $check or $code_check) and the exercise was checked
3. The first event reporting data.completed causes the progress to move forward. (I haven't traced the logic, but testing indicates that the first completed event isn't erased by subsequent non-completion counting events).

The end result is that the user must make one non-empty "Run Code" submission for exercises without grading code or one "Submit Answer" submission for exercises with grading code in order to move forward. We do not, however, require that the answer be correct — in our experience limiting users in this way would cause more frustration than it's worth, since it's easy for mistakes in grading code to accidentally make "correctness" impossible.

One other limitation is that if the exercise has initial code, we don't impede progress if the user submits the initial code as it was provided. I think doing so would ultimately lead to issues down the line but I'd be open to revisiting that policy now that I understand the flow better.

learnr-improve-exercise-completion-demo.mp4

[rossellhayes]

LGTM!