Release 5264: Whitelist Facebook user agent

This release contains a change to the default Nginx whitelist that makes Facebook requests for displaying certain images or details related to web content, such as the title of the webpage or the embed tag of a video exempt from the standard bot ratelimit. The FPM slot limit still applies. Users can further configure their ratelimiting settings in the Nginx config in /data/web/nginx. This change will decrease the chance of previews on Facebook not loading or a 429 being cached. This change will be deployed on all Hypernodes over the course of the coming week.