This adds a type table for broadcast providers, which is the pattern we
follow with our models (e.g. we have a `broadcast_channel_types` table).
As well as the four providers, the migration populates it with `all`
which is the value that will replace `null` in a later change.
It should be safe to add the foreign key constraint to the
`service_broadcast_settings` in the same migration since the column is
still nullable and we don't have data in that column that is not in the
types table.
aec631f208