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

webroot/rsrc/css/layout/phabricator-source-code-view.css

@@ -30,6 +30,10 @@
   user-select: none;
 }
 
+.phabricator-source-highlight {
+  background: #ff0;
+}
+
 .phabricator-source-code-summary {
   padding-bottom: 8px;
 }