[REF] website_slides: rework slide's public and embedded views

Purpose
=======

A slide's 'Embedded views' are reworked as 'Public views' to simplify
the model and be more compliant with the new 'is_preview' field.

The rule is now the following:

A slide is a preview or the view is done as public user (once per session)
-> increment public_views

A slide is viewed as a logged in user that has access to the content
-> increment slide_views
Technically slide_views is a compute of the slide_slide_partner count.

Tests are also added to ensure it is effectively working.

This commit is a sub-part of task ID 1940360 (eLearning integration with
survey) and task ID 1902304 (main eLearning task).