Add support for raw HTML editor.

CHANGELOG.rst

cms/djangoapps/contentstore/features/html-editor.feature

cms/djangoapps/contentstore/features/html-editor.py

cms/templates/widgets/html-edit.html

common/lib/xmodule/xmodule/html_module.py

common/lib/xmodule/xmodule/js/fixtures/html-edit-visual.html

common/lib/xmodule/xmodule/js/fixtures/html-edit.html

common/lib/xmodule/xmodule/js/fixtures/html-editor-raw.html

common/lib/xmodule/xmodule/js/spec/html/edit_spec.coffee

common/lib/xmodule/xmodule/js/src/html/edit.coffee

common/lib/xmodule/xmodule/templates/html/raw.yaml

lms/templates/widgets/html-edit.html