feat: featured image support for inline popups

[adekbadek]

All Submissions:

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

Changes proposed in this Pull Request:

Building on #924, adds featured image support for inline popups, too.

How to test the changes in this Pull Request:

1. Switch Newspack Theme to feat/prompt-styles branch (feat: remove prompt-related styles newspack-theme#1887)
2. Create a new prompt, observe a featured image can be added
3. View the frontend, observe the featured image is displayed
4. Observe the featured image is centred in a 200px high element above prompt content

Other information:

• 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?
• Have you successfully ran tests with your changes locally?

[dkoo]

Looks like this got superseded by #924