[HttpKernel] Move the bundle booting into a separate kernel method

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

We are currently working on using Symfony in the next [Contao][1] major version. We need to extend the boot process, which is currently only possible by overriding the whole `boot()` method. With this change, we could simply extend the `bootBundles()` method:

```php
// AppKernel.php
public function bootBundles()
{
    $this->doOurStuff();
    parent::bootBundles();
}
```

[1]: https://contao.org