redact media when redacting event

[jason-famedly]

• Prevent visibility of media when event has been redacted
• Physically remove media when redacted event is actually censored
• Allow moderators/system admins an opportunity to view said media before it is censored

[jason-famedly]

When it is time to censor an event, it happens on two occasions:

1. when ephemeral events are being removed(some how this relates to deactivating users as well)
2. when an event is redacted and it's retention period is over

It runs as a looping call on the CensorEventsStore and only on the background worker