Ignore internal users on user counts

What does this MR do?

Adds User.real that finds active non-internal users.

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

The easier alternative would be to change active to ignore non-internal users. I wasn't sure which way to go, but it would be confusing if .active does filter by more unexpected things (and we may want to know if a "bot" has been active somehow).

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