[FIX] website_slides: hide publish widget if user cannot publish

Purpose
=======

The "publish/unpublish" widget is always shown on slide.channel/slide.slide for
a website_publisher user even if he's not allowed to manage the publishing of
the channel (see slide.channel.can_publish field).

This commit will hide the widget on both the slide.channel and the slide.slide frontend
pages if the user doesn't have the right to publish/unpublish. A generic solution
implemented directly in the publish mixin has been chosen to ease understanding
and allow easy reuse in other applications.

Commit linked to task ID 1902304 (main elearning task).

closes odoo/odoo#31187