Automatically generate keys when missing

[davidak]

Manually generating keys causes a lot of issues (#81), especially when the documentation is not correct!

To make it more straight forward, the keys should get automatically generated (as suggested in #28 (comment)), so the user does not have to do anything.

I could add this in the systemd service, but i think it would be better to add it to the software directly, so it does also work when used not on nixos. I know several programs that does that like Caddy and BOINC.