[FIX] web_editor: prevent dropping form fields outside of their form

Drop zones are available outside form when dragging form fields.
This leads to errors whenever the field is dropped outside a form.

This commit removes all drop zones outside the form when a field is
dragged.

Steps to reproduce:
- Edit Contact Us form.
- Drag'n'drop a field to the footer.
=> Error popup (should not be allowed in the first place)

task-2855670

closes odoo/odoo#94029

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