Display a warning if a new Privacy/Cookie Policy Page is about to be created but one already exists

[jazzsequence]

We can't really know if a page is a privacy/cookie policy page if it's not set (depending on the post title is unreliable) but if a policy page is set and saved as an option, clicking the Create {Cookie/Privacy} Policy Page button will create a new page and set the active cookie/privacy policy page to the new one.

We can avoid confusion and user error if we display a warning or an alert that says "A {cookie policy/privacy policy} page already exists, are you sure you want to create a new one?" when the button is clicked and a cookie/privacy policy page already exists and an option is saved.

Acceptance criteria:

• [ ] A warning is displayed before creating a new (potentially duplicate) cookie or privacy policy page

[roborourke]

How about not outputting the "Create Cookie/Privacy Policy Page" button if one is already selected? Or changing it to an "Edit page" link if one is set?

[jazzsequence]

Either of those are possibilities 👍