Add "seats in use" table to billings page

What does this MR do?

Adds a paginated table that lists the users occupying the seats in use for a group. This table is displayed in the billings page, underneath the subscriptions table.

• under feature flag api_billable_member_list

This is the implementation MR for this issue: #216899 (closed)

Screenshots

The setup for this screenshot has 7 users per page. In production, there'll be 20 users per page.

Video

The setup for this video has 7 users per page. In production, there'll be 20 users per page.

Screen_Recording_2020-10-16_at_12.01.31

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team