Non-NixOS Hydra installation [needs better documentation]

Hi,

I thought I'd document my experience installing Hydra on a non-NixOS system (Ubuntu 18.04.3):

• I did the multi-user install, it went seemingly fine (under my user)
• manual still has the adding the channel, I only needed to run nix-env -i hydra (under my user)
• manual has something about "Note $prefix ..." but "$prefix" isn't mentioned anywhere else
• manual isn't clear on the createuser and createdb steps (I know nothing about PostgreSQL and very little about SQL in general), got a colleague to do that for me, turns out they needed to be run under the postgres user
• From this point onwards, everything else is done under the hydra user I added for it
• I needed to do https://github.com/NixOS/hydra/issues/584
• For some reason I needed to do systemctl start nix-daemon.service, not sure if this is because something I have done wrong during the installation, I did try systemctl enable nix-daemon.service but it doesn't seem to work. This is probably something broken with my Ubuntu? [edit: actually, it is possibly because I have /nix as a mount, and systemd thinks the file nix-daemon.service, a symlink to /nix/var/nix/profiles/default/lib/systemd/system/nix-daemon.service is broken]
• manual only mentions hydra-server, hydra-evaluator, and hydra-queue-runner. For declarative jobs, I needed to start hydra-notify. There is also hydra-send-stats which always seems to die, on my NixOS declaratively defined hydra too (Died at /nix/store/zhbxmfmp8w0b8kgl831pm9970is8lnja-hydra-2019-08-30/bin/.hydra-send-stats-wrapped line 14)

This is related to improving hydra documentation https://github.com/NixOS/hydra/issues/210 but I decided to make it a new issue to make it easier to find.

Start with the Hydra manual linked in the issue and compare its non-NixOS installation section with the reported Ubuntu 18.04.3 steps. Review the PostgreSQL createuser/createdb instructions, nix-daemon startup, and required Hydra services including hydra-notify and hydra-send-stats. Done means the manual clearly documents the necessary non-NixOS setup and service commands.