Add gzip to nginx server config. (#5700)

* Add gzip and expiration to nginx server config.

* Add application/json to gzip_types line to cover translations.

* Add gzip_vary for content caches, remove expires section.

Per discussion, expiration seems likely to cause more confusion than it solves.  Add gzip_vary to prevent content caches from caching un-compressed versions of the content and confusing browsers.

doc/debian/jitsi-meet/jitsi-meet.example

     index index.html index.htm;
     error_page 404 /static/404.html;
 
+    gzip on;
+    gzip_types text/plain text/css application/javascript application/json;
+    gzip_vary on;
+
     location = /config.js {
         alias /etc/jitsi/meet/jitsi-meet.example.com-config.js;
     }