[IMP] website_slides: allow to remove attendees

PURPOSE

Improve attendees management by adding a button to remove them in backend
of eLearning.

SPECIFCIATIONS

In order to easily remove a attendee from a channel, we add a button in
the slide_channel_partner tree view, accessible from channels (attendees).
Remove a slide_channel_partner implies to remove the related slide_slide_partner.
Also code backtracking for the view of attendees of a course: we redirect
to the slide.channel.partner model instead of res.parter because
we want the possibility to create/remove/edit attendees for a
course and not making action on res.partner model itself.

Task ID 2045620