RFC 235: Add code intel postgres container

Created by: efritz

This is part of RFC 235: Move LSIF data to Postgres.

Update the following environments to configure a new database container, and ensure that the frontend has credentials to the new database. This needs to be done before we change code to read the new variables.

• server environment
• pure docker
• docker-compose environment
• k8s template environment
• configure postgres in cloud environment

This issue must follow https://github.com/sourcegraph/sourcegraph/issues/13912.