Add cookieValidationKey to request config

When I create app using samdark/yii2-minimal template and try to run app, I get error:

`yii\web\Request::cookieValidationKey must be configured with a secret key.`

In `config/web.php` I add config for `request`:

```php
'request' => [
	// !!! insert a secret key in the following (if it is empty) - this is required by cookie validation
	'cookieValidationKey' => 'thisIsAKey',
],
```

After that there was no error.