nixos/mailman: increase uwsgi buffer size

otherwise results in 502 bad gateway errors with some clients (which send a lot of cookies?)

Change-Id: I9aadedb7acde0388f060dbb82ccd8788f41ff0e6
(cherry picked from commit 4d8e8de)

nixos/modules/services/mail/mailman.nix

@@ -595,6 +595,7 @@ in {
             plugins = ["python3"];
             home = webEnv;
             http = "127.0.0.1:18507";
+            buffer-size = 8192;
           }
           // (if cfg.serve.virtualRoot == "/"
             then { module = "mailman_web.wsgi:application"; }