Fix #348, #474, #557, #740, #928: improved deduplication logic to prevent skipped files during import

Resolve "When using docker-compose deployment, the celerybeat container may stumble over a stale pid file"

Advertise list of known nodes on /api/v1/federation/domains and in nodeinfo if stats sharing is enabled

This reverts commit 7b0db234.

Also fix the duplicate track add confirmation margins
Fix #974

Fix #947: Make media and static files serving more reliable when reverse proxy X_FORWARDED_* headers are incorrect