Convert the app's configuration system from a parsed YAML file to
12-factor style envvars overriding defaults. Envvar based config is the
preferred Toolforge solution since the introduction of the `toolforge
envvars` service for managing environment variables to inject into
Kubernetes workloads.

A typical deployment will want to set the SINKS_ENABLED, SOURCE_TOKEN,
PHABRICATOR_TOKEN, and GITLAB_TOKEN envvars. The LOG_EVENTS_TO and DEBUG
envvars may also be useful. See lib/settings.py for more details on
these values and to see the system defaults.

Bug: T362288