When generating a new page key, it was only made sure to not match
existing page keys. This leads to COW happening on existing views if the
key already existed in a view.

This commit ensures that new page keys are not existing view keys
either.

Steps to reproduce:
- Create a page named "snippets".

=> Notification was shown indicating that `website.snippets` is private.

task-3328827

closes odoo/odoo#122669

Signed-off-by: Romain Derie (rde) <rde@odoo.com>