table. Also add created_by_api_key_id column that will supersede
api_key_id_column.
We do migration and code in separate PRs to make it easier to
downgrade if anything goes wrong.
Add check constraint that created_by_id should not be null, unless
created_by_api_key_id is not null to the migration script. It is
already in the models file.
Also remove check constraint for cancelled_by_id from models, as
this field would only be filled for broadcasts with cancelled
status.
Also add some spacing in that migration script so it is easier
to read.
