[FIX] web_editor: fix drop zones when dragging a snippet

Before this commit:

-when moving a snippet using the drag and drop button, two drop zones
instead of one was added at the initial location of the dragged
snippet.

-when moving a snippet using the drag and drop button, no drop zone
was added at the initial location of the dragged snippet if the snippet
was alone in its parent. For example, the left jumbotron in the banner
snippet.

-the drop zones handled by the "children rules" were never vertical
when it was necessary.

This commit also refactors the code to avoid some duplicated lines.

task-2312878

closes odoo/odoo#61336

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>