[ext] More flexible layout for recos on Youtube, from 1 to 6 videos per row

[amatissart]

The behavior below format has been proposed.

Youtube's video per row	Number of reco (expanded)	Number of reco (default)	Number of rows (expanded)	Number of rows (default)
n	2n	n	2	1
6	12	6	2	1
5	10	5	2	1
4	12	4	3	1
3	12	3	4	1
2	12	4	6	2
1	12	2	12	2

I think this can be implemented by putting all 12 videos in the same wrap block with the right width, but such that videos that overflow are not visible. Then adapting only the height of the recommendation box.

Originally posted by @lfaucon in #1667 (comment)