Add unenrollment capability

[gsidebo]

We would like to use the API client to unenroll a user from a course in edX

[gsidebo]

During some local testing a while back, I ended up using the un-enrollment endpoint directly via HTTP. Here is a snippet of that code:

### Enroll via HTTP API
usernames = ['honor', 'audit', 'staff']
pw = 'edx'
course_id_str = 'course-v1:MIT+gsidebo000+2017_Summer'
enroll_url = '/change_enrollment'
#
from django.contrib.auth.models import User
from django.test.client import Client
def set_enrollment(course_id_str, usernames, pw, enroll=True):
  client = Client()
  enroll_url = '/change_enrollment'
  for username in usernames:
    login_resp = client.post(
              '/user_api/v1/account/login_session/',
              data={
                  "email": "{}@example.com".format(username),
                  "password": pw,
                  "remember": "false"
              },
              headers={
                  'Referer': '/login',
              },
    )
    enrollment_action = 'enroll' if enroll else 'unenroll'
    resp = client.post(enroll_url, data={'course_id': course_id_str, 'enrollment_action': enrollment_action})