Change to event-level beforeRedaction event for efficiency

To avoid an O(n^2) situation with every relations container trying to process
every redaction that may occur in a room, this switches to an event-level
notification, so that the specific relations container who cares can listen to
just the events it wants to know about.

src/models/event.js

@@ -661,6 +661,8 @@ utils.extend(module.exports.MatrixEvent.prototype, {
             throw new Error("invalid redaction_event in makeRedacted");
         }
 
+        this.emit("Event.beforeRedaction", this, redaction_event);
+
         // we attempt to replicate what we would see from the server if
         // the event had been redacted before we saw it.
         //