tuesmiddt

Add foreign key constraints to model changsets

2

Where models have foreign keys that cannot be null, the tables should have appropriate constraints and changesets should also validate this with `Ecto.Changeset.foreign_key_constraint/3`. See [here](https://elixirforum.com/t/ecto-association-vs-foreign-key-constraints/2387/2) for difference with `Ecto.Changeset.assoc_constraint/3`. Todo:...