Add HTML editor for authoring bulk email message (adopted from CMS)

common/static/css/tiny-mce.css

lms/djangoapps/instructor/views/legacy.py

lms/envs/common.py

lms/static/img/problem-editor-icons.png

lms/static/sass/base/_variables.scss

lms/static/sass/course.scss.mako

lms/static/sass/course/instructor/_email.scss

lms/templates/courseware/instructor_dashboard.html

lms/templates/widgets/html-edit.html