CMS-859 Disable page privacy settings for CMS users

[AdamHawtin]

What is the context of this PR?

We need to disable Wagtail's page privacy features. This PR achieves this by overriding the can_set_view_restrictions permission test in the BasePagePermissionTester to always return false, preventing any CMS admin UI user from setting view restrictions for pages.

It also includes a migration to remove any existing page view restrictions as we are not supporting them and CMS users will no longer be able to remove them themselves after this change.

How to review

Run this branch, then try accessing the page privacy settings for any page in the Wagtail admin UI, as an admin, publishing admin and publishing officer. You should not be presented with the options to change page privacy settings.

Deployment Safety

Bleed and Sandbox deploy automatically on merge, so PRs should be safe to deploy immediately.

Please select one:

• Safe to auto-deploy
• Not safe to auto-deploy

Follow-up Actions

List any follow-up actions (if applicable), like needed documentation updates or additional testing.

[AWitcherONS]

Unit tests pass
page privacy not seen

[MaciekBaron]

LGTM

Question: Should we consider adding a migration removing any existing restrictions, for cleanup purposes?

[sanjeevz3009]

LGTM

Question: Should we consider adding a migration removing any existing restrictions, for cleanup purposes?

@MaciekBaron @AdamHawtin It's mentioned in the JIRA card:

We may need to consider doing a mop up of any pages that
have been made private in exisitng environments, since users
would not be able to make them public again after this change.

Maybe it needs a separate card as a follow-up? If so good to have a card created and chucked into the backlog.

[sanjeevz3009]

Tested it against all page types. Works well. LGTM!

[AdamHawtin]

LGTM

Question: Should we consider adding a migration removing any existing restrictions, for cleanup purposes?

Good point, I've added a migration in d1da0aa.

[MebinAbraham]

Other than Sanjeev's comment. 👍

[sanjeevz3009]

LGTM!