Switch back to zwsp for oncopy line marker

Summary:
Fixes T4759.

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