RM1849: Auto-generating security keys

Can you please review the attached patch, and apply if you're happy with it?

The purpose is to auto-generate the various security keys that are currently in the configuration file, and store them in the SQLite database. This allows us to remove the checks for config_local.py and the hard-coded default keys which are causing some problems with packaging:

- Hard coded defaults are fine for Desktop mode, and packages generally aim to make that work primarily.

- Hard coded defaults are a security risk for Server mode, hence we currently require the user to manually setup keys, which is currently being overridden by packagers for Desktop mode.

This change ensures that we have unique security keys for every installation, whether running in desktop or server mode (generated from os.urandom).

Thanks!