CollectionItem primary key should be its relations (item and user)

[FOSSforlife]

Right now its primary key (as with all data models that implement Warthog's BaseModel) is an autogenerated ID. We want the primary key to instead be the item and user that the CollectionItem is connected to, because 1) the combination of these is unique, and 2) we want to be able to easily and quickly fetch these CollectionItems by their item and user IDs.