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.
