Before this commit, if you had a page_id set on a menu, and that you
fork this page, the page_id from the menu was always pointing to the
previous one, so you see old page, but edit new page.
> Reproduce:
> Fork Contactus (Page_id on menu become wrong)
> Refresh contact us (done on save btw)
> You see old content
> Edit with Html Editor, you sill see that forked view exists!
Bug was hidden before that we set page_id on menu, but it is needed if
we want to support the groups visibility on the menu item.
closesodoo/odoo#39472
Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>