Implement core redaction logic (#1185)

* Implement core redaction logic

- 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.

* Linting

roomserver/storage/sqlite3/event_json_table.go

@@ -35,8 +35,7 @@ const eventJSONSchema = `
 `
 
 const insertEventJSONSQL = `
-	INSERT INTO roomserver_event_json (event_nid, event_json) VALUES ($1, $2)
-	  ON CONFLICT DO NOTHING
+	INSERT OR REPLACE INTO roomserver_event_json (event_nid, event_json) VALUES ($1, $2)
 `
 
 // Bulk event JSON lookup by numeric event ID.