[FIX] website: disable mega menu hover mode while editing

When a mega menu "Sub Menus" are configured as "On Hover", it becomes
very difficult to edit its content.

This commit changes the behavior of the "On Hover" while the page is
being edited:
- it disables the hide on exit (`mouseleave`)
- it prevents the show on hover if another dropdown is already opened
- it hides the menu when the page is clicked outside of the opened menu

This commit also disables the preview of the "Show Sign In" option
which is not previewed anyway, so that it does not deselect the mega
menu when hovering the options.

task-2825376

Part-of: odoo/odoo#110258