Make "show code" a button. Clean up feedback template and display logic. [Finishes #68317004]

Show code button:

![image](https://cloud.githubusercontent.com/assets/206973/2770856/a5f5cfa4-ca63-11e3-8488-362f9276aa88.png)

![image](https://cloud.githubusercontent.com/assets/206973/2770871/ceebbedc-ca63-11e3-94b0-24b63ab74feb.png)

Flappy sharing spacing (unchanged, now `min-height` div rather than `<br/>`):

![image](https://cloud.githubusercontent.com/assets/206973/2770862/b223e248-ca63-11e3-81ba-073c8e1d4d05.png)