[IMP] website_slides_forum : set communication on slide.channel at creation - frontend side

Since 556b6454dab07cdf716d404ec0db9aa51a09b165, channel can be linked to a dedicated forum.
But the link could only be done in backend, in the slide.channel configuration
For now on, website_publishers can create a new dedicated forum when they create
a new slide.channel.

Link buttons have been added in both forum and slide.channel headers to navigate easily
between them.

Fix the relation between course and forum in a model point of view.

Task ID : 1922159
PR #31109