Switch back to zwsp for oncopy line marker
Turns out Chrome on windows doesn't really like the word joiner character. We'll switch back to zwsp but make it position: absolute; so it doesn't turn into a line break.
Test Plan: Looked at diffs in IE9 and Chrome Windows. Made sure copying still works as expected.
Reviewers: chad, Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: epriestley, Korvin
Maniphest Tasks: T4759
Differential Revision: https://secure.phabricator.com/D8727