[IMP] website_slides: allow comments on slides

With this commit, the user can activate review on channel content when
creating a channel from frontend, or editing it from backend. Once
activated, the comment tab below the slide content will allow user
to post a comment with optional rating.
The way of commenting is different regarding channel type:
- documentation: user can post comment and vote (list/dislike) on
slide content
- training: user can post comment with review (rating)

Task-1902304