Forums new post form should reset state when user cancels

TNL-148
![a](https://cloud.githubusercontent.com/assets/4027165/4125848/9e7d91b8-32e5-11e4-95af-b2e24e0ade8c.png)
![b](https://cloud.githubusercontent.com/assets/4027165/4125849/9e7e34ec-32e5-11e4-8cd8-7fdf5879ca0b.png)