Change Cancel button styling to btn-link style

Fixes #640 

Just changes ```btn-default``` to ```btn-link``` to make Cancel buttons styled as links.

![curation_-_add_exhibit_contact___default_exhibit_-_blacklight](https://cloud.githubusercontent.com/assets/101482/2713210/1a47f53e-c4e3-11e3-89fa-7ca7ee846223.png)