Exclude Pending Memberships from Billable Members Count

What does this MR do and why?

Exclude pending memberships from the billable members count. This addresses #345694 (closed).

This change also handles pending group memberships for some cases, so it also applies to the work for #332598 (closed).

See !75554 (closed) for the full extent of the changes to handle pending memberships for #332598 (closed). !75554 (closed) will be broken down into small MR's to make database review and so forth as easy as possible.

The database queries are commented on here: !79377 (comment 827231918)

Issue: #345694 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.