BLD-368: Turn "download transcript" into a dropdown.

CHANGELOG.rst

...ontentstore/features/component_settings_editor_helpers.py

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

cms/static/js/views/metadata.js

cms/static/sass/views/_unit.scss

common/lib/xmodule/xmodule/tests/test_video.py

common/lib/xmodule/xmodule/video_module.py

lms/djangoapps/courseware/tests/__init__.py

lms/djangoapps/courseware/tests/test_video_mongo.py

lms/djangoapps/courseware/tests/test_video_xml.py