[REF] website_slides: improve duration computation of a slide/course

The completion time of a slide now has default values for slides of type

  * video (duration of the YouTube video);
  * PDF (number of pages * 5 minutes);

This computation is done when uploading a slide in frontend or by an onchange
in backend. Field can still be given / modified by elearning people as
result may not be reliable (pdf parsing is not accurate, and/or you may
estimate time differently from strict page-based computation).

The course will sum up all durations of its (active) slides. Display has been
changed from "x minutes" to "x hours y minutes".

Task ID 2007294