nixos/pleroma: add pleroma NixOS module

This module is not trying to configure either postgresql nor nginx.
It's is a design decision, not an omission. A webserver setup can be
highly complex.

The idea is trying not to be smarter than the user, providing them
with a simple tool. They are smart enough to figure the interaction
between the various component by themselves!

The module has one and only job: setting up a pleroma service.

modules/pleroma.nix

@@ -81,7 +81,7 @@ in {
         ExecStartPre = pkgs.writers.writeBashBin "pleromaStartPre" ''
           if [ ! -f "/etc/pleroma/config.exs" ]; then
             echo "ERROR: Missing pleroma config file at /etc/pleroma/config.exs"
-            echo "Did you read http://git.xxxxx ?"
+            echo "Did you read https://git.alternativebit.fr/NinjaTrappeur/pleroma-otp-nixos/src/branch/master/readme.md#user-content-pleroma-configuration-management ?"
             exit 1
           fi
           ${cfg.package}/bin/pleroma_ctl migrate'';