Checks user access permissions for paths under aff4:/users.

def _IsHomeDir(self, subject, token):
    """Checks user access permissions for paths under aff4:/users."""
    h = CheckAccessHelper("IsHomeDir")
    h.Allow("aff4:/users/%s" % token.username)
    h.Allow("aff4:/users/%s/*" % token.username)
    try:
      return h.CheckAccess(subject, token)
    except access_control.UnauthorizedAccess:
      raise access_control.UnauthorizedAccess(
          "User can only access their "
          "home directory.", subject=subject)