[IMP] website_slides: allow to download slide content and additional resources

PURPOSE

Currently eLearning lacks feature of having downloadable resources on
lessons. Indeed for example teachers would like to attach zip files or
other resources linked to a given lesson.

SPECIFICATIONS

You can now allow the user to download the slide content for pdf and image
slides. A new icon will be visible next to the fullscreen button on the pdf
viewer.

Course maintainers can also add additional resources to a slide which will be
available to download for the user.

Task ID 2066741
PR #39890

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