[IMP] website_slides: indicate new content on courses to users

Purpose of this commit is to help users knowing there is new content in a
course by adding a visual insight. It is done through a new content arrow
displayed in courses homepage.

Regarding the file "website_slides_templates_homepage.xml", the choice to
incorporate the t-call attribute into a 't' balise was necessary to display
the customize option (part front) associated with the model course_card.

Side dish usability improvements raised during development

  * display completed courses as last instead of first in "my courses";
  * do not show promote strategy field for training courses as it has no
    use, only for documentation courses;

TASK ID 2025186
Closes PR #36703

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>