[UserCap BE]: Billable members without pending members

Why are we doing this work

In &4315 (closed), we introduced a User Cap setting for self-managed admins to prevent accidental user overages, now we are doing the same for SaaS group owners to prevent accidental overages.

This work will be broken down into the following backend steps:

• Add user cap setting
• Adding/inviting members
• Notifications
• Member management
• Auto approval
• Approval Modal
• Group access modifications
• Group sharing
• UserCap availability
• Billable members 👈🏽 we are here

For this issue, we should prevent members in a pending/awaiting state from being considered billable for a group.

Relevant links

• Discussion around this requirement: #332596 (comment 725450369)

Implementation plan

• Modify billable member methods to exclude awaiting members e.g. here and here

We should consider the use of a feature flag for this given it's implication on billing

Additional notes

• The Member class already defines an active scope which relates to the User state, not the Member state, here, which may be confusing
• This change will likely need involvement from the database team for discussion around query performance/plans
• We will also need to update the documentation around this area to be clear that pending/awaiting members are not billable. If it's not already there by the time this issue is worked on, we could add a new section to Groups for User Cap related docs
• We should consider the impact of this change on related classes such as GitlabSubscription with regards to the seat usage calculations