Add Feedback form

Closes https://github.com/sul-dlss/spotlight/issues/824 and via that, https://github.com/sul-dlss/spotlight/issues/791

Adds standard SUL feedback form to app.

![stanford_university_libraries__online_exhibits](https://cloud.githubusercontent.com/assets/101482/5496129/f3eda904-86b8-11e4-8388-31040d7f7b08.png)