[FIX] website_slides: fix resequencing based on category

When new slides are added through frontend a resequencing of slides and
categories sequence is done by calling _resequence_slides on the channel.
However this method is not working as expected, and resequencing should
take a category in order to better know where to start its resequencing.
Otherwise it simply reorder slides in the same order as before, or in an
unpredicted or prodicted but fasly order if new slide category has just been
created.

Task ID 2090927

X-original-commit: f038946ae2e3f174bee086281a541c6bc770e354