Add friendsofsymfony/user-bundle as a requirement of the Core Component

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Hi,

Currently, the `friendsofsymfony/user-bundle` dependency is part of the `required-dev` section of the `Core` 's composer.json, but the class `Sylius\Component\Core\Model\Group` extends `FOS\UserBundle\Model\Group` which makes it a `require`.