Embed pastes now support highlight

Summary:

• Added support for highlighting to PhabricatorSourceCodeView
• Added support for highlighting to embed pastes

Test Plan: {F35975}

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1770

Differential Revision: https://secure.phabricator.com/D5346