s4f-conference: allow larger uploads

they should probably be using nextcloud for this, but i can't be
bothered to make them a group there, so here we go 🤷

parsons/s4f-conference.nix

@@ -126,6 +126,8 @@
         proxy_hide_header Content-Security-Policy;
         proxy_hide_header X-Frame-Options;
         proxy_redirect off;
+
+        client_max_body_size 100M;
       '';
     };
     forceSSL = true;