Legacy email tests, removed duplicate code, updated comments, fixed CSS

CHANGELOG.rst

common/djangoapps/terrain/course_helpers.py

common/djangoapps/terrain/ui_helpers.py

lms/djangoapps/instructor/features/bulk_email.feature

lms/djangoapps/instructor/features/bulk_email.py

lms/djangoapps/instructor/tests/test_api.py

lms/djangoapps/instructor/tests/test_email.py

lms/djangoapps/instructor/tests/test_legacy_email.py

lms/djangoapps/instructor/views/api.py

lms/djangoapps/instructor/views/instructor_dashboard.py

lms/envs/acceptance.py

lms/envs/common.py

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

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

lms/templates/courseware/instructor_dashboard.html

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