Update pleroma.nginx

proxy_ignore_client_abort should be enabled when using mediaproxy and an nginx cache.

Leaving it out could lead to broken media in the cache due to connections broken halfway or other issues which would lead to nginx not properly loading the file from upstream.

With proxy_ignore_client_abort nginx will not abort upstream connections when client connections are aborted. In other words it will finish fetching and caching the media file from mediaproxy regardless of if the client still wants it or not. Avoids cache corruption.