Properly enable gzip compression in nginx config

Given that there's already a bunch of other gzip options set in the
nginx config, it's likely someone forgot to actually add the setting to
turn gzip on, so let's fix that.

installation/nginx/akkoma.nginx

@@ -58,6 +58,7 @@ server {
     ssl_stapling on;
     ssl_stapling_verify on;
 
+    gzip on;
     gzip_vary on;
     gzip_proxied any;
     gzip_comp_level 6;