API_URL is still accepted for BC compatibility of the self-hosted deployments.
This will allow deploying on a single domain, using a reverse proxy that acts on the path.
This however means that the config needs to be changed for all WorkAdventure deployments.
Without this patch, the index.html contains google analytics at
all times, including for people self-hosting it.
This is a problem for privacy reasons, and only people wanting
to have analytics on their instances should be able to enable
them.
This fixes it by making sure the index.html page is templated
to sideload content from ANALYTICS_CODE_PATH (which itself is
also templated, for convenience).
