code: function deleteUser(username, done) {
    return getUser(username, function(getUserErr, user) {
      if (getUserErr) {
        return done(getUserErr);
      }
  
      if (!user) {
        return done(new Error(`user '${username}' not found`));
      }
  
      user.tokens.forEach((token) => token.destroy());
      user.groups.forEach((group) => { group.members.remove(user.id); group.save(); });
      user.leading.forEach((group) => { group.leaders.remove(user.id); group.save(); });
      user.destroy(done);
    });
  }
docs: Destroy a user. This also deletes all the user's tokens.
  
  @param {String} username
  @param {Function} done - done(err)