Create auto cohorts when list_cohorts is called and return number of users per cohort.

TNL-186
Add support for UX reference templates in LMS
Merge pull request #5133 from edx/christina/cohorts-api

Create auto cohorts when list_cohorts is called.
Merge pull request #5176 from edx/andya/lms-ux-reference-templates

Add support for UX reference templates in LMS