Introduces new database environment variables while keeping
backward compatibility with the now deprecated
POSTGRES_ENV_POSTGRES_USER POSTGRES_ENV_POSTGRES_PASSWORD
environment variables.
DATABASE_URL defaults to an auto generated url using the
`DATABASE_HOST`, `DATABASE_PORT`, `DATABASE_USER`,
`DATABASE_PASSWORD` and `DATABASE_NAME` variables.
Part-of: <https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/2313>
jo