Implement MSC3912: Relation-based redactions #14231
Description
This issue has been migrated from #14231.
Implement support for automatically redacting messages based on their relation with a message that's being manually redacted, as specified in MSC3912.
- Redact related events that have already been sent: Implement MSC3912: Relation-based redactions matrix-org/synapse#14260
- Redact events sent after the redaction request completes