Line highlighting for pastes

Summary:
Add the ability to select singular and multiple lines in paste to highlight.
This is related to T3627

Test Plan: Create a paste, select one or more lines.

Reviewers: epriestley, tberman

Reviewed By: epriestley

CC: aran, chad

Maniphest Tasks: T3627

Differential Revision: https://secure.phabricator.com/D6668