add request membership button to discussion show

screenshots : 

![image](https://f.cloud.github.com/assets/2665886/1536939/0c819c34-4cbb-11e3-900f-03044707154f.png)


![image](https://f.cloud.github.com/assets/2665886/1536957/76b4bd5c-4cbb-11e3-84e3-f45f72f0171e.png)