Modified request method for change enrollment and bulk beta modify access on instructor dashboard.

LMS-11266

Request method changed to POST for change enrollment and bulk beta tester modify access because GET request is failing when try to enroll lots of students at one time.

Tests fixed.
use [`require_post_params`](https://github.com/edx/edx-platform/blob/master/lms/djangoapps/instructor/views/api.py#L145-L179)