Add None option to site policies

[enricobattocchi]

Context

Summary

This PR can be summarized in the following changelog entry:

• Changes the default value for the site policies options to 0 and adds None as a selectable option in the settings.

Relevant technical choices:

• Important: this sets the default value to 0 while it used to be false. since the feature has not been released yet, no upgrade code has been added. It's very important then to test this PR on a clean environment where the values has not been set yet.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• install on a clean environment (see above the technical reason)
• Install and activate the latest Premium RC
• Go to Yoast SEO -> Settings -> Site representation
• Select organisation and add name and logo.
• Visit Site basics -> Site policies and check they are all set to None by default
• Check the Schema in the home page and see that you don't see any site policy property
• Visit Site basics > Site policies once more
• Set one or more fields to a page and save
  • While you are there, check the autocomplete works as expected
  • Check that if you empty the field, you get the a list of pages + None which is pre-selected and gets displayed even if you click outside the control
• Check the Schema in the home page and see that you see the expected site policy properties
• Visit Site basics > Site policies once more
• Set one or more fields to None and save
• Check the Schema in the home page and see that you see the expected site policy properties (the ones, if any, which you didn't set back to None

Relevant test scenarios

• Changes should be tested with the browser console open
• Changes should be tested on different posts/pages/taxonomies/custom post types/custom taxonomies
• Changes should be tested on different editors (Block/Classic/Elementor/other)
• Changes should be tested on different browsers
• Changes should be tested on multisite

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

QA can test this PR by following these steps:

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Other environments

• This PR also affects Shopify. I have added a changelog entry starting with [shopify-seo], added test instructions for Shopify and attached the Shopify label to this PR.

Documentation

• I have written documentation for this change.

Quality assurance

• I have tested this code to the best of my abilities.
• During testing, I had activated all plugins that Yoast SEO provides integrations for.
• I have added unit tests to verify the code works as intended.
• If any part of the code is behind a feature flag, my test instructions also cover cases where the feature flag is switched off.
• I have written this PR in accordance with my team's definition of done.
• I have checked that the base branch is correctly set.

Innovation

• No innovation project is applicable for this PR.
• This PR falls under an innovation project. I have attached the innovation label.
• I have added my hours to the WBSO document.

Fixes #https://github.com/Yoast/ux/issues/50

[vraja-pro]

CR & AC ✅