feat(footer): update template part to use desktop and mobile parts

[thomasguillot]

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 uses the same concept as the Header, where the main template part uses 2 "sub" template parts: 1 for desktop and 1 for mobile. These parts then contain a pattern that a publisher can choose from.

The reason for this change is that it doesn't always make sense to have the same footer on both mobile and desktop. For example, some sites have a very complex footer on desktop with many links, which doesn't scale well on mobile. By having two template parts, we give publishers more control over what they want to display depending on the screen size.

Note: For now, there's only one style for the mobile footer. However, we might add more in the future.

How to test the changes in this Pull Request:

1. Switch to this PR
2. Switch to a different theme and then switch back to the Block Theme (I noticed some caching issue with patterns in the past and this switch theme "trick" seem to work)
3. Navigate to the Footer template part and see if you see the 2 sub template parts
4. Edit the sub template parts, either by switch pattern or by just modifying the content
5. Save and check your front-end (large and small screen)

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?

[adekbadek]

Works as described, but the resulting HTML produces a footer within a footer, which is not a semantic use of HTML.

[matticbot]

🎉 This PR is included in version 1.13.0-alpha.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[matticbot]

🎉 This PR is included in version 1.13.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀