Implement persist / flush in repository

[migo315]

Maybe we should implement persist, remove and flush in repositories. So we don't have to inject the EntityManager only for a flush if we already injected a repository:

Current:

    public function __construct(
        DocumentManagerInterface $odm, # Only for persist and flush
        ProductRepository $productRepository,
    ) {
        $this->odm = $odm;
        $this->productRepository = $productRepository;
    }

Target:

    public function __construct(
        ProductRepository $productRepository,
    ) {
        $this->productRepository = $productRepository;
    }