add Mattermost service configuration and related variables

systems/palatine-hill/mattermost.nix

@@ -0,0 +1,19 @@
+{
+  config,
+}:
+let
+  vars = import ./vars.nix;
+in
+
+{
+  services.mattermost = {
+    enable = true;
+    siteUrl = "https://mattermost.nayeonie.com"; # Set this to the URL you will be hosting the site on.
+    database = {
+      peerAuth = true; # This allows Mattermost to connect to the database without a password, which is more secure when both are on the same machine.
+      create = true;
+      driver = "postgres";
+    };
+    dataDir = "${vars.primary_mattermost}/mattermost";
+  };
+}