- Add a new `redactions_table.go` which tracks the mapping of
the redaction event ID and the redacted event ID
- Mark redactions as 'validated' when we have both events.
- When redactions are validated, add `unsigned.redacted_because`
and modify the `eventJSON` accordingly.
Note: We currently do NOT redact the event content - it's gated
behind a feature flag - until we have tested redactions a bit more.
