Implemented bulk email interface for new dashboard

lms/djangoapps/instructor/views/api.py

lms/djangoapps/instructor/views/api_urls.py

lms/djangoapps/instructor/views/instructor_dashboard.py

lms/djangoapps/instructor/views/legacy.py

lms/envs/common.py

...ffee/src/instructor_dashboard/instructor_dashboard.coffee

lms/static/coffee/src/instructor_dashboard/send_email.coffee

lms/static/sass/course/instructor/_instructor_2.scss

lms/templates/instructor/instructor_dashboard_2/email.html

...ructor/instructor_dashboard_2/instructor_dashboard_2.html

...mplates/instructor/instructor_dashboard_2/send_email.html