Simplify oncopy behavior

Summary:
Firefox has supported clipboardData since version 22 (Jul 2013), and even IE8 supports it if you look at `window.clipboardData` instead of `e.clipboardData`. As a result, we can simplify this code significantly.

I also used (or at least, attempted to) Javelin so that we can get the event object and preventDefault more easily. Plus, this way we don't assign to document.body.oncopy.

Test Plan: Copied a selection including a line number in Chrome, Firefox, and IE8. The line number didn't get copied.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8688