fix(preview): handle additional_classes

[adekbadek]

All Submissions:

• [x] Have you followed the Newspack Contributing guideline?
• [x] Does your code follow the WordPress' coding standards and VIP Go coding standards?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Fixes a bug where additional CSS classes were not applied in editor previews.

See 1200550061930446-as-1206541498328358

How to test the changes in this Pull Request:

1. On trunk, edit a prompt and set some CSS classes in "Advanced Settings" sidebar panel
2. Trigger a preview, observe the classes are not applied
3. Switch to this branch, try again, observe the classes are applied to the newspack-popup-container element

Other information:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [ ] Have you written new tests for your changes, as applicable?
• [x] Have you successfully ran tests with your changes locally?

[matticbot]

:tada: This PR is included in version 2.31.0-alpha.2 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket:

[matticbot]

:tada: This PR is included in version 2.31.0 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: