nix: implement flake

This commit introduces the ability to install Docspell from a Nix flake.
There are no major changes to the logic of the previous modules outside of
organizing them in a flake and adding a simple check script.

nix/pkg.nix

@@ -1,54 +0,0 @@
-cfg: {stdenv, lib, fetchzip, file, curl, inotifyTools, fetchurl, jdk11, bash, jq, sqlite}:
-let
-  meta = with lib; {
-    description = "Docspell helps to organize and archive your paper documents.";
-    homepage = https://github.com/eikek/docspell;
-    license = licenses.gpl3;
-    maintainers = [ maintainers.eikek ];
-  };
-in
-{ server = stdenv.mkDerivation rec {
-    name = "docspell-server-${cfg.version}";
-
-    src = fetchzip cfg.server;
-
-    buildInputs = [ jdk11 ];
-
-    buildPhase = "true";
-
-    installPhase = ''
-      mkdir -p $out/{bin,docspell-restserver-${cfg.version}}
-      cp -R * $out/docspell-restserver-${cfg.version}/
-      cat > $out/bin/docspell-restserver <<-EOF
-      #!${bash}/bin/bash
-      $out/docspell-restserver-${cfg.version}/bin/docspell-restserver -java-home ${jdk11} "\$@"
-      EOF
-      chmod 755 $out/bin/docspell-restserver
-    '';
-
-    inherit meta;
-  };
-
-  joex = stdenv.mkDerivation rec {
-    name = "docspell-joex-${cfg.version}";
-
-    src = fetchzip cfg.joex;
-
-    buildInputs = [ jdk11 ];
-
-    buildPhase = "true";
-
-    installPhase = ''
-      mkdir -p $out/{bin,docspell-joex-${cfg.version}}
-      cp -R * $out/docspell-joex-${cfg.version}/
-      cat > $out/bin/docspell-joex <<-EOF
-      #!${bash}/bin/bash
-      $out/docspell-joex-${cfg.version}/bin/docspell-joex -java-home ${jdk11} "\$@"
-      EOF
-      chmod 755 $out/bin/docspell-joex
-    '';
-
-    inherit meta;
-  };
-
-}