feat: add large image size for larger featured images

[laurelfulford]

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:

Our theme uses 1200px for featured images, but this isn't really adequate for the large featured image sizes that came after the theme was first built -- specifically, the Behind, Above and sometimes Behind (due to height) image sizes.

This PR adds an additional 2000px size to just use for those cases on single posts and pages. It does mean larger image sizes, but Jetpack's Site Accelerator should help balance that out.

See 1200550061930446-as-1205443589299352

How to test the changes in this Pull Request:

1. Create three posts with the above, behind and beside featured image placements, respectively.
2. Publish, and view on the front-end; note that the image quality isn't great, especially on larger screens.
3. Apply the PR; you will likely also need to regenerate thumbnails just to get your images up to date; you can use this plugin or the CLI command.
4. Refresh your posts and confirm they're loading a larger image size. Note: it still may not be ideal for all images -- like ones that are more illustrative/text heavy -- but it should be an improvement!

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?

[laurelfulford]

Thanks @adekbadek!

[matticbot]

🎉 This PR is included in version 1.87.0-alpha.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[matticbot]

🎉 This PR is included in version 1.87.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀