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_132](https://cloud.githubusercontent.com/assets/4497748/2850750/6cc0b338-d108-11e3-9391-8b91a13379ee.png)
![selection_133](https://cloud.githubusercontent.com/assets/4497748/2850751/6cc4434a-d108-11e3-8e1d-d266e1199222.png)