[IMP] website_slides: update demo data course responsible / attendee

Before
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In the demo data, we manually add ``admin`` as an attendee of his course,
and then we remove it with ``_remove_membership`` because he will be added
automatically in python.

Now
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We do not add ``admin`` as an attendee of the course, and we remove the
``_remove_membership`` call. We also change the responsible of some courses
so admin can join some courses.

Task ID 2120225
PR #39886

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>