Add support for raw HTML editor.

STUD-1562
adjust codemirror window on raw HTML editor
UI String  Updates for Raw HTML Editor Change

Modified the Raw HTML template text and the help string for the Editor
setting.
Tests for raw editor and minor wording changes.
Merge pull request #3449 from edx/hotfix/2014-04-24

Hotfix/2014 04 24 Raw HTML Editor for Studio