[C2] Add course duration

[ywarnier]

The idea of evaluating the time necessary to complete a whole course is becoming more and more popular. Despite Chamilo offering the possibility to have extra fields expressing this, it still requires modifications to templates to actually show that information.

Create a duration field of type int in the course table to be future proof. This field can be null.

This field can be edited. If it is set, it should appear in the data returned by api_get_course_info() as duration.
It should also appear (only if it is not null) in the course catalogue and in the list of courses/sessions as a connected user, but it should be set in a specific div class (course-duration) so we can hide it through CSS if we want.

[ywarnier]

We also need to add an estimated duration to these:

c_survey.duration
c_quiz.duration (only to display/count time) (does not replace expired_time)
c_quiz_question.duration
c_lp.duration
c_lp_item.duration
c_student_publication.duration (requires migration if previously an extra_field 'work_time' existed attached to c_student_publication)
c_attendance_calendar.duration (requires migration of extra_field 'duration' attached to c_attendance_calendar)

[ywarnier]

All good from my point of view !