UX: show category when moving post to exisiting topic

[Meta topic](https://meta.discourse.org/t/move-to-existing-topic-should-show-category/20128).

![screen shot 2014-09-19 at 20 52 17](https://cloud.githubusercontent.com/assets/5732281/4337756/6da508de-4012-11e4-98b9-f4c6cec728fe.png)