migrate to environment variables over site config
Created by: coury-clark
Site config is not trivial to manipulate in managed instances, and is not restricted to Sourcegraph employees (or license checks, for example). This migrates all the required config to environment variables.
Test plan
EXPORT_USAGE_DATA_ENABLED=true EXPORT_USAGE_DATA_TOPIC_NAME=usage-data-testing EXPORT_USAGE_DATA_TOPIC_PROJECT=sourcegraph-dogfood gogo
[ worker] INFO worker.export-usage-telemetry telemetry/telemetry_job.go:196 fetching events from bookmark {"bookmark_id": 21816}
[ worker] INFO worker.export-usage-telemetry telemetry/telemetry_job.go:207 telemetryHandler executed {"event count": 4, "maxId": 21821}