codeintel: RFC 235: Add code intel postgres container

Created by: efritz

A step towards RFC 235: Move LSIF data to Postgres.

Add environment variables to server and local environments to point code intel database connection environment variables at the existing postgres. The new database schema will be designed so that they can be overlays of each other without conflicts. This allows only one database instance required for local and single-container deployments, and multiple-instance deployments in production environments.

Partially implements https://github.com/sourcegraph/sourcegraph/issues/13883 along with sibling PRs https://github.com/sourcegraph/deploy-sourcegraph/pull/896 and https://github.com/sourcegraph/deploy-sourcegraph-docker/pull/153.