feat: add an option to turn the homepage into a no-header no-footer splash page

[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:

This PR adds a Customizer option to make the front page a 'splash page' with no header and no footer - due to how WordPress themes work, we can't just assign the existing template.

It also fixes some styling issues in Newspack Nelson with the No Header No Footer template.

See NPPM-2421 .

How to test the changes in this Pull Request:

1. Apply this PR & run npm run build.
2. Go to Customizer > Homepage Settings, and confirm there's an option to "Make the homepage a splash page", and that it's not checked.

3. Check the option, and click "Publish".
4. View the front page, and confirm there is no header or footer.
5. Edit the homepage, and add a full width block as the first and last block in the content, and add some content in between -- the full width block should be something visible, like a group block with a background, or a cover block.
6. View on the front end, and confirm the first block touches the top of the browser window. The last block should also touch the bottom of the browser window if there's enough content (if you have to scroll to it), and if you're viewing it in an incognito window -- otherwise the Edit link from the template will add a small gap.

7. Create a static page and assign the No Header No Footer template; add similar content (the full-width block at the start and end).
8. View on the front-end and confirm it looks the same as the front page, except the content area is narrower (780px instead of 1200px).
9. Switch the site to Newspack Nelson and retest both your homepage and static page - confirm they look the same and don't have the extra padding/spacing that this theme normally has.
10. Click around on regular posts/pages with Nelson still assigned and make sure things look okay.
11. Switch back to one of the other themes; click around on regular posts and pages and make sure things still look okay.

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 @leogermani!

[github-actions]

Hey @laurelfulford, good job getting this PR merged! 🎉

Now, the needs-changelog label has been added to it.

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Thank you! ❤️

[matticbot]

🎉 This PR is included in version 2.16.0-alpha.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[matticbot]

🎉 This PR is included in version 2.16.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀