Publish lightboxing on the message bus

With this change, images appear to lightbox instantly on my development
machine.
Centralize MessageBus post updates

After this change, only two files directly publish to MessageBus with a
topic interpolated in the channel: Post and TopicUser.
Remove unnecessary user/topic load in PostCreator
Merge pull request #2705 from riking/live-lightbox

Publish lightboxing on the message bus