Issue 37264: allow turn on audit logs

Created by: filiphaftek

Description

Part of issue

Allow enabling audit logs (security_event_logs) in any Sourcegraph instance via environment variable (required for audit managed instances): AUDIT_LOG_ENABLED=true. Change is backward compatible, as flag SOURCEGRAPHDOTCOM_MODE=true also turns on security_event_logs.

Test plan

Run unit tests. Tested on local SG dev.