feat: add a UI to add the non editing contrib role

[leogermani]

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:

Adds a UI to assign users to the custom Non-Editing Contributor role:

How to test the changes in this Pull Request:

1. Edit a user
2. See the new "Co-Authors Plus" section
3. Check the box to add the custom role save
4. Confirm the role is added to the user
5. Uncheck the box and save
6. Confirm the role is removed
7. Confirm you don't see this section when you edit your own profile

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]

Nice! Much better than having to recommend a multi-user-role plugin.

[leogermani]

closed in favor of #3277 See pemhSX-Sz-p2