[FIX] web: consider overflow visible dimensions for draggable

In a Kanban View, the container in which elements are dragged is a `d-flex`
element, and as such it has the `overflow: visible` css property by default.
This means that if there are more groups than the current dimensions of the
viewport allows for, those groups will overflow outside the dimensions of the
container, preventing elements from being dragged outside.

Therefore the dimensions of the draggable area should consider the container
`scrollWidth` and `scrollHeight`, as these values take into account the
dimension of overflowing elements.

task-3339978

closes odoo/odoo#122576

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