Add save-method to RelationFactory

[andersbjorkland]

Add a missing save-method to RelationFactory to streamline it with ContentFactory. This method will persist the Relation entity or collection of Relation entities.

[andersbjorkland]

I'm working on this now. It's almost a carbon copy of ContentFactory->save($content). I'm currently writing some unit tests for it.

There is one change to the class besides adding the save-method. This is the constructor that now will take two arguments:

    public function __construct(RelationRepository $repository, EntityManagerInterface $em)
    {
        $this->em = $em;
        $this->repository = $repository;
        $this->relations = collect([]);
    }

One could argue that $repository is accessible from the EntityManager, but I don't want to affect any other code that uses this constructor. As such, I'm tacking on the EntityManager parameter at the end of the constructor so code that want to use the save-method adds this on creating the RelationFactory (or let dependency injection do it for them).