Fix nginx caching issues

Nginx is currently not caching data because proxy_buffering needs to be
enabled for caching to work at all, and we are receiving a Cache-Control
header from Pleroma that states "max-age=0, private, must-revalidate"

Even disregarding the Cache-Control header that should actually be set
to "public, max-age=1209600" as defined in the reverse_proxy code, we
don't want to obey this header at all as it overrides our Nginx caching
rules.

installation/pleroma.nginx

@@ -79,8 +79,10 @@ server {
         proxy_cache_valid  200 206 301 304 1h;
         proxy_cache_lock on;
         proxy_ignore_client_abort on;
-        proxy_buffering off;
+        proxy_buffering on;
         chunked_transfer_encoding on;
+        proxy_ignore_headers Cache-Control;
+        proxy_hide_header Cache-Control;
         proxy_pass http://localhost:4000;
     }
 }