Simplify how underscore.string is referred to.

common/static/common/js/components/views/feedback_alert.js

...tatic/common/js/components/views/feedback_notification.js

common/static/common/js/components/views/feedback_prompt.js

common/static/common/js/utils/edx.utils.validate.js

lms/envs/common.py

lms/static/js/certificates/models/certificate_exception.js

...static/js/certificates/models/certificate_invalidation.js

lms/static/js/spec/main.js

lms/static/js/student_account/views/AccessView.js

lms/static/js/verify_student/views/step_view.js

lms/static/lms/js/build.js

lms/static/lms/js/require-config.js

lms/templates/commerce/checkout_receipt.html

lms/templates/verify_student/incourse_reverify.html

lms/templates/verify_student/pay_and_verify.html

lms/templates/verify_student/reverify.html