Bring back the "cancel" button to node forms.

#5905 

This PR brings back the cancel button on all node types as requested in https://github.com/openscholar/openscholar/issues/5905#issuecomment-41829999

Output:
![selection_118](https://cloud.githubusercontent.com/assets/4497748/2850013/e3a755b0-d0ef-11e3-98f8-5842877feaba.png)