discourse: FIX: Also allow 21m in dockerized nginx.

So there's 3 place to configure max upload body size:
- Discourse settings (Via admin web interface)
- Host nginx (via Ansible)
- Guest nginx (Via app.yml)

roles/discourse/templates/app.yml

@@ -6,6 +6,7 @@
 ##
 ## After making changes to this file, you MUST rebuild
 ## /var/discourse/launcher rebuild app
+## Rebuild WILL cause ~10mn of downtime.
 ##
 ## visit http://www.yamllint.com/ to validate this file as needed
 
@@ -21,6 +22,7 @@ params:
   db_shared_buffers: "256MB"
   db_work_mem: "40MB"
   version: tests-passed
+  upload_size: "21m"
 
 env:
   LANG: fr_FR.UTF-8