Boilerplate SignupForm for advanced application template

Current `SignUp` form have 2 issues:

- its return value should be boolean because its used in boolean context:

```php
            if ($user = $model->signup()) {
                if (Yii::$app->getUser()->login($user)) {
                    return $this->goHome();
                }
            }
```

- if `User` model contains some additional rules for affected fields and `$user->save()` will return `false`, non valid user will be logged in.