Create invitations for MyJobs accounts [PD-542]

`FACTORY_FOR` is deprecated. This should be:
```python
class InvitationFacdtory(factory.django.DjangoModelFactory):
    class Meta:
        model = Invitation
```

We could also remove the import (thus preventing possible circular imports) by using a string instead:
```python
class Meta:
    model = "registration.Invitation"
```