hydra-module.nix: Automatically create postgres db user for hydra and an admin hydra account

The initial password for the admin account can be found in /var/lib/hydra/.pgpass.

Signed-off-by: Shea Levy <shea@shealevy.com>
Don't put ssmtp in hydra's paths

Some installations may want to use system-wide sendmail (i.e.
/run/setuid-wrappers/sendmail) and those that want ssmtp can add it to
hydra's path themselves.

Signed-off-by: Shea Levy <shea@shealevy.com>
Add an option to run the hydra server in debug mode

Signed-off-by: Shea Levy <shea@shealevy.com>