This diff adds the detect_base_url config, controlled by the
DETECT_BASE_URL env variable. When set to true, the BASE_URL setting is
ignored, and the base_url is derived from the request protocol and host
header.
Test Plan: Started up a local instance in my homelab, running docker
node:15 image with a nginx reverse proxy. Configured nginx to use the
same backend with multiple hostnames on https. Opened in browser and
confirmed og:url meta tag uses correct url.
02e8cb264f