CLC-1724 Add the X-Frame-Options header
https://jira.media.berkeley.edu/jira/browse/CLC-1724

- secure_headers gem can provide several security headers but we're only using x-frame for now
- See Jira for screenshots showing this in action
- Left comments noting that this can be removed when we migrate to Rails 4
Merge pull request #1126 from shacker/CLC-1724

CLC-1724 Add the X-Frame-Options header