Convert main.js to CoffeeScript

Created by: rspeicher

Does a bit of refactoring along the way, removing methods that are no longer required or used, and fixes a misspelling of "empty" in a function name.

Also adds two jQuery extension functions: enable and disable which removes some duplication we were doing with disabling form fields and adding the 'disabled' class to them at the same time. The two methods do what you'd expect.