Rework how hiding messages works. (#1509)

* Rework how hiding messages work. Fixes #1350

* Remove unused function

* Revert to old event name to support previously saved webhooks

core/chat/events/setMessageVisibilityEvent.go

@@ -0,0 +1,21 @@
+package events
+
+// SetMessageVisibilityEvent is the event fired when one or more message
+// visibilities are changed.
+type SetMessageVisibilityEvent struct {
+	Event
+	UserMessageEvent
+	MessageIDs []string
+	Visible    bool
+}
+
+// GetBroadcastPayload will return the object to send to all chat users.
+func (e *SetMessageVisibilityEvent) GetBroadcastPayload() EventPayload {
+	return EventPayload{
+		"type":      VisibiltyUpdate,
+		"id":        e.ID,
+		"timestamp": e.Timestamp,
+		"ids":       e.MessageIDs,
+		"visible":   e.Visible,
+	}
+}