Patch the bulk_enroll request to use a form content type

Description

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

*Dependencies*: None

*Sandbox URL*: TBD - sandbox is being provisioned.

*Testing instructions*:

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.

*Reviewers*

  • [ ] @pomegranited

  • [ ] edX reviewer[s] TBD

*Settings*
```yaml
EDXAPP_FEATURES:
ENABLE_COMBINED_LOGIN_REGISTRATION: true
ENABLE_BULK_ENROLLMENT_VIEW: true
```

Done

Assignee

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Reporter

Open Source Pull Request Bot

Contributor Name

Brandon DeRosier

Repo

edx/edx-platform

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