Add our own base model
We should add our own base model, that extends pubsweet's base model.
All our other models should extend our custom base.
Our own custom base would of course define all methods that we want all models to have.
For the time being, let's just add a
findById method that calls objection's
findById, if only to have sth to test against.
Where this gets tricky is when one of our models extends one of pubsweet's other than base (eg. TeamMember extends pubsweet's TeamMember, not base).