feat: persistent logging for the broker

[ckerr]

We currently keep the broker's log in memory, but that's not ideal -- it means the logs will disappear forever if the broker gets restarted.

Whatever persistence solution we use needs to play nice with Heroku, since that's where the broker is deployed.

Previous discussions have been redis or mongodb. Whoever claims this issue gets to decide and do it the way they like!

This is related to (and possibly a duplicate of) #52

[MarshallOfSound]

If the logs are just an HTML file / text file the cheaper solution to a full on database is to just dump the logs on an S3 bucket somewhere. (We can make an S3 bucket on our AWS account, or you can provision one through heroku afaik but AWS would be cheaper lol)