greenlight: increase presentation upload limit

BBB itself already configures a 30MB limit, but this hadn't been reflected in
greenlight config so far. With the recent pre-upload feature being gated
through greenlight, this inconsistency became visible.

templates/greenlight.nginx.j2

@@ -9,6 +9,8 @@ location /b {
   proxy_set_header    X-Forwarded-For   $proxy_add_x_forwarded_for;
   proxy_set_header    X-Forwarded-Proto $scheme;
   proxy_http_version  1.1;
+  # allow uploading presentations up to 30MB
+  client_max_body_size  30M;
 }
 
 location /b/cable {