Fix: Database migration and mail template files were not included in dist.

* Moved copying server-build/ from grunt to package.json
  using rsync to make sure to copy exactly what is needed
  and resolve symlinks correctly.

shell.nix

@@ -10,6 +10,7 @@ stdenv.mkDerivation rec {
     git
     nasm
     nodejs-10_x
+    rsync
     sqlite
     zlib
   ];