Assessment refactoring and question block in answer

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@antoviaque Actually, there are several reasons:

1. Blocks without a `name` (such as all floating blocks) will never be submitted.
2. Even if they have a `name`, they also need to have a `submit` method.
3. In case of assessment, only the active_child is checked for submission which is always a step (and thus never an HTML element or a floating block.)

See code: https://github.com/edx-solutions/xblock-mentoring/blob/master/mentoring/public/js/mentoring_assessment_view.js#L156-L168