[FIX] website_slides{_*}: fix completion computation, notably with certifications

PURPOSE

Test frontend and UI tools of eLearning.

SPECIFICATIONS

Since 9920f20e course completion when finishing a certification survey
is broken. Indeed channel completion is not correctly recomputed. A
certification course can therefore never be completed and awarded. This
commit fixes that and correctly update channel completion based on survey.

In this commit we also rename some methods

  * _compute_completion: renamed to _recompute_completion to avoid naming issue
    with computed field convention;
  * _post_completion_hook is not unnecessary as code in hr_skill can be done
    after calling super of _recompute_completion;
  * _completed_callback is renamed to _set_completed_callback to be coherent
    with slides naming;

LINKS

Task ID 1937768