When we have a public API there will be no human creating the broadcast
message. Instead it will be created by an API integration, authenticated
by a key (just like for emails or texts).
This updates the database to:
- add a new foreign key from BroadcastMessages to API keys
- add a `reference` column
It doesn’t change the model yet, because the model is used by previous
migrations, so would cause them to fail when run before the new columns
exist. We can make this change in later pull requests.
