add broadcast_event table

It's clear that we need a way to track updates to a broadcast message.
It's also clear that we'll need some kind of audit log that captures
exactly what was sent out in a message.

This commit adds a new database table, `broadcast_event`, which maps 1:1
with CAP XML sent to the CBCs. We'll create one of these just before
sending out.

The main driver for this was that cancel and update messages need to
contain a list of references of all previous messages that they're
amending. This is of format `{sender},{identifier},{sent_timestamp}`,
and the identifier itself needs to be unique for each message.

tests/conftest.py

@@ -117,6 +117,7 @@ def notify_db_session(notify_db, sms_providers):
                             "organisation_types",
                             "service_permission_types",
                             "auth_type",
+                            "broadcast_status_type",
                             "invite_status_type",
                             "service_callback_type"]:
             notify_db.engine.execute(tbl.delete())