[FIX] website: Editing the URL of a menu with an already existing URL

Before this commit:
If you changed the URL of a menu linked (m2o) to a website page and the new URL
was already taken by another page, it would edit the menu.url and the
menu.page.url resulting in the menu's page having the same URL than the already
existing one.

Now, if you edit the URL of a menu linked to a page and the URL is already
taken by another page, we will simply change the menu/page m2o relationship
without writing on the menu's initial page's URL.