Feat/change nicename UI

[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:

This is an experimental UI to #3725.

This is not meant to be enabled for every user, but it can be enabled if there's a special need for this feature.

How to test the changes in this Pull Request:

1. add define( 'NEWSPACK_CHANGE_NICENAME_UI', true );
2. Edit a user
3. Confirm there's a new section called "Change User archive URL"
4. Check test instructions on feat: handle user nicename change #3725 and perform them, but using the UI this time

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?