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/postgres/storage.go

@@ -91,6 +91,10 @@ func Open(dataSourceName string, dbProperties sqlutil.DbProperties) (*Database,
 	if err != nil {
 		return nil, err
 	}
+	redactions, err := NewPostgresRedactionsTable(db)
+	if err != nil {
+		return nil, err
+	}
 	d.Database = shared.Database{
 		DB:                  db,
 		EventTypesTable:     eventTypes,
@@ -106,6 +110,7 @@ func Open(dataSourceName string, dbProperties sqlutil.DbProperties) (*Database,
 		InvitesTable:        invites,
 		MembershipTable:     membership,
 		PublishedTable:      published,
+		RedactionsTable:     redactions,
 	}
 	return &d, nil
 }