[FIX] website_slides : fix and add tests on karma gain

Fix karma channel completion computing and karma gain :
    set to complete only if not completed
    and handle eventual multi course completion
Removes unused karma gain rules.
Add karma unvote rule to loose the karma gained if user changes his mind.
Test karma gain on each users.
    Note : technically, public user could be member of a course,
    but functionally speaking, this case should and could never happen
    (it's a non sens anyway)
    -> public user has not been tested for this reason.

Task ID : 1943788
Closes PR #31321

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