[IMP] website: button to easily back to page manager from form view

Before this commit:
When you were on the page manager and you were editing a website.page's
content, it would redirect you to the website.page's ir.ui.view's form view in
the backend.
From there, you would'nt have an easy way to get back to the page manager:
your only option was to go to the website module and click on 'go to website'
and then click on menu/manage pages.
It was not convenient when you were editing multiple page quickly or just
taking a look into some pages.

Now, you have a button on the ir.ui.view's form view to get directly on the page
manager.

Note: this back button is obviously invisible for ir.ui.view not related to a
website.page