Generate secret key on first run

[thieman]

The default config file currently has a scary WARNING flag letting the user know to create their own secret key. This will probably get overlooked in a bunch of cases, leading to uncustomized Dagobah installations all using the same key.

I think a better solution would be to generate a new secret key the first time Dagobah is run, replace the secret key value in the config file, then put an indicator somewhere (possibly in the config file itself) so we can track whether any given run is the first run.