Revert "Revert "Track viewing of individual blocks.""

...module/assets/vertical/public/js/vertical_student_view.js

common/lib/xmodule/xmodule/library_content_module.py

common/lib/xmodule/xmodule/tests/test_vertical.py

common/lib/xmodule/xmodule/vertical_block.py

lms/envs/aws.py

lms/envs/common.py

lms/static/completion/js/.eslintrc.js

lms/static/completion/js/ViewedEvent.js

lms/static/completion/js/spec/ViewedEvent_spec.js

lms/static/karma_lms.conf.js

lms/static/lms/js/spec/main.js

lms/templates/vert_module.html

webpack.common.config.js