[FIX] web: should keep background of folded column

**Issue:**

When moving a record in a kanban view from one column to another,
it is possible to keep holding the record, hover it in one column
for a moment, then move again to another column then eventually
drop the record. Doing this where a folded column is the intermediate
column results to visual issue, such that the darker highlight of
a folded column is lost.

The desired behavior is to keep the background color of a folded
column.

**Solution:**

When moving a record, the hovered column is highlighted, and right
after leaving the column, we remove the highlight. And we are doing
this without exempting the folded column, thus, its darker highlight
is lost during the record-moving-hovering process.

Instead of adding/removing .bg-100 during unhover/hover on each
kanban column to make sure of the "greenish" tint of the column when
hovered, we just make background of the .o_kanban_hover to be important.

closes odoo/odoo#105557

Signed-off-by: Julien Mougenot (jum) <jum@odoo.com>