The original version of the `gen-passwords.sh` script is
part of docker-jitsi-meet:
https://github.com/jitsi/docker-jitsi-meet/blob/master/gen-passwords.sh

The version here is taken from tag `stable-9779`, last
modification in commit ID `084266cec54854373d039d20eff13318b2903580`.

Local modifications:
* none; importing the original version unchanged so we can track all of our changes in git.