Auto generate Biff secrets + save them to config.edn?

[jacobobryant]

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:biff/jwt-secret and :biff.middleware/cookie-secret are nil by default in config.edn.TEMPLATE. Normally when you create a new Biff project, the project creation script generates some random secrets for you. But when cloning an existing Biff project (like Platypub) that doesn't happen. So I think whenever people start up the app, they're getting a new random set of secrets? Which would mean they have to sign in every time. (I have secrets myself since I'm the one who generated the Platypub project!)

We should add instructions for generating secrets and add them to the readme. Maybe even add a command to do it all automatically if it's not too much work.