Kind-of-terrible (?) oncopy handler

Summary: Works in Safari, Firefox, Chrome.

Test Plan: Copied some text, threw up a little in my mouth.

Reviewers: aran, tuomaspelkonen, tomo, rstout, btrahan

Reviewed By: aran

CC: aran, epriestley, ddfisher

Maniphest Tasks: T145, T995

Differential Revision: https://secure.phabricator.com/D244