Line count can be set for paste

Summary:
Probably not the ideal way to deal showing only certain amount of lines. Size vary by browsers,
zooming will mess it up albeit only a little and will definitely not work with IE.

Test Plan: {F35989}

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1770

Differential Revision: https://secure.phabricator.com/D5347