Ignore internal users on user counts

What does this MR do?

Counts non-internal users only for active users filtering

Are there points in the code the reviewer needs to double check?

I first went for creating a separate scope for this on https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1658 but then I changed my mind, since we may only want to show active non-internal.

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/2096

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together