Feat/user counts

[adekbadek]

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 two features, which will be useful when auditing user synchronisation:

1. New column in the WP Users view, featuring origin site URL and User ID on that site (the text is a link to the user edit screen):

2. New column in the Nodes table, displaying synchronisable users count:

How to test the changes in this Pull Request:

1. Visit WP Users tables on the Hub and Node sites, verify the new column features correct data and links
2. Visit the Nodes table on the Hub site, verify the user counts match
3. Click the user counts, observe the links point to WP Users table filtered by synchronized roles

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]

Superseded by #79