Add client_max_body_size configuration to nginx-proxy

Technically I could use a per-host configuration here and forego the
whole nextcloud nginx container, but for the time being it's simpler
to set the global upload limit to 16G as well.

etc/nixos/services/configs/nginx-proxy.conf

@@ -0,0 +1 @@
+client_max_body_size 16G;

etc/nixos/services/nginx.nix

@@ -9,6 +9,7 @@
         "443:443"
       ];
       volumes = [
+        "${./configs/nginx-proxy.conf}:/etc/nginx/conf.d/general.conf:ro"
         # So that we can watch new containers come up
         "/var/run/docker.sock:/tmp/docker.sock:ro"
         # So that we can access generated certs