This PR adds two optional columns to the bulk register/enroll csv: cohort (WIP) and course mode. This enables setting the course mode and cohort for a user in the same process as bulk enrolling/registering.
*JIRA tickets*: TBD
*Sandbox URL*: TBD - sandbox is being provisioned.
*Merge deadline*: None
1. set `ALLOW_AUTOMATED_SIGNUPS: true` under `FEATURES` (automatically set on the sandbox)
2. navigate to the membership tab of the instructor dashboard for a test course
3. upload a csv following the existing format, but with two extra colums (cohort name and course mode)
4. verify that correct course mode and cohort names have been set.
5. TBD: there are multiple edge cases; more test instructions to come once the behaviour has been implemented and stabilised.
*Author notes and concerns*:
[ ] @sspj
[ ] (openEdx reviewer TBD)