batch ingest should not duplicate objects

[ebenenglish]

This is a placeholder ticket, more detail TK

We want to prevent re-running a batch ingest (in case there is an error/failure first time around) from re-creating existing objects.

There is some existing functionality for preventing duplicate NewspaperTitle objects from being created (see #175), but nothing currently exists for Container, Issue, Page, Article, etc.

To make this work, we'll need to identify some metadata attributes (or combination of attributes) that can be used to create a unique value for the above objects so that the batch ingest script can test if they already exist.

This is essentially just a typical Rails find_or_create pattern.