Make TinyMCE more tolerant about HTML elements

STUD-1565
fixes templating error (LMS-2560)
Remove verify_html as it isn't supported in TinyMCE 4
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 #3460 from edx/release

Merging release back to master