feat(homepage-posts): sortable post selection

[miguelpeixe]

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:

Make post selection for Homepage Posts sortable using https://github.com/clauderic/react-sortable-hoc

homepage-posts-sortable.mov

How to test the changes in this Pull Request:

1. While on the master branch, build a page using the Homepage Posts block with specific posts selection
2. Check out this branch, edit the same page and confirm the selection is preserved
3. Sort using the drag handle, save, refresh and confirm the order was preserved
4. Delete and add new items and confirm it behaves as expected
5. Add a new Homepage Posts block with specific posts selection and confirm it also works with a fresh block
6. Visit the page and confirm the same order is also rendered
7. Confirm the default component behavior is also preserved by adding a new Homepage Posts block and querying through authors and categories

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?

[yogeshbeniwal]

findDOMNode method used by https://github.com/clauderic/react-sortable-hoc is due for removal in a future major version of React.

[miguelpeixe]

Thank you for the heads up, @yogeshbeniwal!

The library is also no longer actively maintained. It's a bummer, being such a lightweight straightforward solution. I'll revisit this PR soon.