Add support for configurable primary keys on ORM models

[jlowin]

Summary

As we make heavier use of views, the assumption that every model has a corresponding id key is more likely to be validated. This PR makes each ORM model's primary key configurable. It does not replace existing uses of the word "id" as an argument (models.Flow.where(id=5)) because the headache would be... too much. However, models.CustomFlow.where(id=5) would work even if CustomFlow's primary key were a field other than id.

Importance

Checklist

This PR:

• adds new tests (if appropriate)
• adds a change file in the changes/ directory (if appropriate)

[codecov-io]

Codecov Report

Merging #145 (adfc8fb) into master (02911e9) will increase coverage by 0.21%.
The diff coverage is 100.00%.