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

@@ -72,15 +72,13 @@ in {
         # It's sub-optimal as we'll always run this, even if pleroma
         # has not been updated. But the no-op process is pretty fast.
         # Better be safe than sorry migration-wise.
-        ExecStartPre =
-          let preScript = 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 https://git.alternativebit.fr/NinjaTrappeur/pleroma-otp-nixos/src/branch/master/readme.md#user-content-pleroma-configuration-management ?"
-              exit 1
-            fi
-            ${cfg.package}/bin/pleroma_ctl migrate'';
-          in "${preScript}/bin/pleromaStartPre";
+        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 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'';
 
         ExecStart = "${cfg.package}/bin/pleroma daemon";
         ExecStop = "${cfg.package}/bin/pleroma stop";