[IMP] website_slides: improve and add statistics computation

Purpose of this commit is to add some statistics computation on slide.channel
model. We want notably to have a count of views and votes (likes and dislikes)
on channels. Completion is also computed.

Purpose is to be able to search and order channels based on those statistics.
Having stored computed fields for some statistics help achieving that purpose.

Tests are added.

This commit is linked to task ID 1936153 and PR #30985.