LMS: revising .is-hidden utility class to be used at a global level
Merge pull request #5422 from edx/talbs/cohort-management-is-hidden

Cohort Management: Making .is-hidden class globally accessible
Support adding cohorts from the instructor dashboard

TNL-162
Fix test failure
Move underscore into base vendor requirements.
Implement adding cohorts
Bok choy test for adding a cohort.
Complete implementation
Address code review comments.