Add foreign key constraints to model changsets

[tuesmiddt]

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 for difference with Ecto.Changeset.assoc_constraint/3.

Todo:

• Update changesets
• Add migrations

[tuesmiddt]

For the assessments context, this will be complete when #144 is merged.
Not done yet for accounts/course contexts.

[martin-henz]

I asked Julius, and this is what he says:

Essentially we didn't really use course and accounts contexts so we haven't put them in

Someone just needs to check that the foreign key constraint has been added