Configure global settings via environment variable

Created by: beyang

Site config and critical site config are currently configurable via environment variables. $CUSTOMER has requested global settings also be settable from environment variable:

as now we are controlling our configs via version controlled repo, we want everything we set, get up and running at the restart of docker container.

suppose we add a new repo in external services and need to add it in quick links. Even though docker restart with new external service config via ENV variable will pick new repo, we still need to add quicklink for it manually.