Consolidate hostname/domain configuration

[metasoarous]

Problem:
Right now there are 5 different environment variables for configuring the main server hostname:

• API_DEV_HOSTNAME
• API_PROD_HOSTNAME
• DOMAIN_OVERRIDE
• EMBED_ SERVICE_HOSTNAME
• SERVICE_URL

For a production deployment, at least 3 of these (API_PROD_HOSTNAME, DOMAIN_OVERRIDE and EMBED_SERVICE_HOSTNAME) are required, and should always be set to the same value. Meanwhile, SERVICE_URL is only used for client-report, and only necessary if hosted separately from the API service. Further, API_DEV_HOSTNAME and API_PROD_HOSTNAME serve the same function, just in different environments, and behave the same, except that for DEV, can default to localhost if unset.

Suggested solution:

• Unify API_DEV_HOSTNAME and API_PROD_HOSTNAME to a single configuration variable (possibly using DEV_MODE to determine whether to allow localhost as a default if unset, to match current behavior)
• Call this new variable API_HOSTNAME? POLIS_HOSTNAME? SERVER_HOSTNAME? Just HOSTNAME?
• Remove the other configuration variables, in favor of afore mentioned merged variable

Additional context:

• At the moment, it's unclear whether SERVICE_URL is only necessary for dev, when you might want to run a HMR server separately, outside of docker, or if it's necessary if you use (e.g.) s3 to host static assets separately from the docker (or other server) deployment infrastructure. Should ascertain this to decide whether this variable needs to be removed or not, and if not, whether another name would be more appropriate.
• Consider whether we want a graceful fallback to API_PROD_HOSTNAME if whatever the new variable name is called isn't present (probably with a deprecation warning), to avoid this being a breaking change.