We want to exclude the system user from group user counts, since intuitively admins wouldn't include them.
Originally this was accomplished by booting said system user from the groups, but this is causing problems, because the system user needs TL group membership to perform certain tasks.
After this PR, system user is still in the TL groups, but excluded when refreshing the user count.
Users who can access the review queue can claim a pending reviewable(s) which means that the claimed reviewable(s) can only be handled by the user who claimed it. Currently, we show claimed reviewables in the user menu, but this can be annoying for other reviewers because they can't do anything about a reviewable claimed by someone. So this PR makes sure that we only show in the user menu reviewables that are claimed by nobody or claimed by the current user.
Internal topic: t/77235.
