Delete (quarantine) media when it's redacted

[turt2live]

Synapse issue: matrix-org/synapse#1263

There's no spec for redactions deleting media, so one may have to be created. When the homeserver detects that a media object is dereferenced everywhere it should contact the media repo and ask it to delete the file. The homeserver should ignore the response code entirely (200, 404, etc are all valid) as it would be just a suggestion, and the repo may not implement it.

The homeserver would be responsible for tracking remotely media being redacted as well. This is to prevent the world from recommending deletion on the media repo. A shared secret auth on the API would probably be enough to verify the right homeserver is contacting the repo.

This may be possible to do with the new pluggable storage layer in synapse develop. In theory, a shim could be written to proxy the calls to us, bypassing synapse.

[turt2live]

This might be possible by hooking into the Synapse replication and database to figure out which data to actually quarantine.