Disable context menu on resource url for download

We have a button that does work masquerading as a link to a file.  This results in an issue with uPortal's naming convention of URL's.  Example from what the URL is on this link: `http://localhost:8080/portal/f/u28l1s4/p/PayrollInformation.n94/normal/irs_statement.pdf.resource.uP?pP_docId=3`.  Normally we can add some info in the context header telling the browser to expect a file returned and file name to save it as.  Safari does not honor this when saving from the context menu (menu that appears when a user right clicks something).
This commit adds a class that can be used to disable the context menu for certain elements.
`class="contextMenuDisallowed"`


This class is added to the link shown below.  It will prevent users in Safari from bring up the context menu and clicking save-as.
![image](https://cloud.githubusercontent.com/assets/5521429/2842782/45b760e2-d078-11e3-80c8-ca86f4e6b094.png)

Browser detection and functionality tested with Mac: Safari, Chrome, Firefox. Linux: Chrome, Firefox. Windows: Chrome, Firefox, Internet Explorer.