Enable private rating

[lenhoanglnh]

Like on the website: https://tournesol.app/rate

[lenhoanglnh]

[tchaumeny]

I tried implementing this but I ended up with a weird behavior in the my_ratings_are_private endpoint (https://tournesol.app/api/v2/videos/my_ratings_are_private/?video_id=ecqtrDTfDm4).

It always yields:

{
    "my_ratings_are_private": false,
    "entry_found": false
}

cc @sergeivolodin

[tchaumeny]

Also, I find the feature not very intuitive from a user point of view: why do we have to set our rating mode for each video ?

It seems more intuitive to have a public/private rating mode switch in the user account, and then to set it once and for all.