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.

tests/conftest.py

@@ -120,7 +120,8 @@ def notify_db_session(notify_db, sms_providers):
                             "broadcast_status_type",
                             "invite_status_type",
                             "service_callback_type",
-                            "broadcast_channel_types"]:
+                            "broadcast_channel_types",
+                            "broadcast_provider_types"]:
             notify_db.engine.execute(tbl.delete())
     notify_db.session.commit()