Clarify the configuration for Read-only DB replicas

[ballPointPenguin]

Problem: Previously there was an optional var DATABASE_FOR_READS_NAME that served as a pointer to another value which could be managed programmatically to point to one of potentially many read-only databases. In the great configuration refactor, this was replaced with a simple READ_ONLY_DATABASE_URL var which is more limited and must be explicitly set just once.

Suggested solution: We need to decide if and how to support multiple read replicas in a way that works in Heroku and non-Heroku environments.

Alternative suggestions: TBD

Additional context: This came up during review of another PR https://github.com/compdemocracy/polis/pull/1663