Boilerplate SignupForm for advanced application template

Current `SignUp` form model 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.