Add a type table for broadcast providers

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.

app/models.py

@@ -2553,7 +2553,7 @@ class ServiceBroadcastSettings(db.Model):
     channel = db.Column(
         db.String(255), db.ForeignKey('broadcast_channel_types.name'), nullable=False
     )
-    provider = db.Column(db.String, nullable=True)
+    provider = db.Column(db.String, db.ForeignKey('broadcast_provider_types.name'), nullable=True)
     created_at = db.Column(db.DateTime, nullable=False, default=datetime.datetime.utcnow)
     updated_at = db.Column(db.DateTime, nullable=True, onupdate=datetime.datetime.utcnow)
 
@@ -2564,6 +2564,12 @@ class BroadcastChannelTypes(db.Model):
     name = db.Column(db.String(255), primary_key=True)
 
 
+class BroadcastProviderTypes(db.Model):
+    __tablename__ = 'broadcast_provider_types'
+
+    name = db.Column(db.String(255), primary_key=True)
+
+
 class ServiceBroadcastProviderRestriction(db.Model):
     """
     TODO: Drop this table as no longer used