[FIX] website_slides: fix display of lessons in training course

Currently slides are displayed in a reorderable list, using current user
access rights. This means external customers see only published and free
lessons.

However purpose of course page is to display all lessons, and display
only available ones as clickable. Other one should be displayed as muted
to tease people.

Note that a specific lesson page does check for access rights. Here we
just display all lessons names. A label for free preview is also added.

Fetch of data when displaying a channel is therefore a bit modified as
there are less differences between training and documentation channels.
User progress is also updated to be easier to use in templates.

Commit linked to task ID 1941250 and PR #31453