feat(membership-plans): display active memberships count

[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 an "Active Members" column in Membership Plans table:

How to test the changes in this Pull Request:

1. On the Hub site, visit the "Membership Plans" view, observe a new "Active Members" column
2. Visit the corresponding Node sites, confirm the numbers are correct

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Superseded by #79