Patch the bulk_enroll request to use a form content type
The plot thickens with with yet another follow-up fix (related to https://github.com/edx/edx-platform/pull/15584). It turns out that the line removed there was necessary for older versions of DRF. I neglected to notice that buried in the response that I received back during testing was actually an error from the downstream `student_update_enrollment` view. Because that view directly pulls the parameters out of POST data, certain conditions need to be met in order for DRF to pull POST data from the "_data" dict.
From my inline comment:
> Setting the content type to be form data makes Django Rest Framework v3.6.3 treat all passed JSON data as
> POST parameters. This is necessary because this request is forwarded on to the student_update_enrollment
> view, which requires all of the parameters to be passed in via POST parameters.
*JIRA tickets*: OSPR
*Sandbox URL*: TBD - sandbox is being provisioned.
Follow the exact same testing instructions that are listed in #15579, except be sure to double-check that the response does NOT contain an error field.
[ ] @pomegranited
[ ] edX reviewer[s] TBD