Adding command for manually "refunding" the user's enrollment

[jkachel]

Pre-Flight checklist

• Testing
  • Code is tested
  • Changes have been manually tested

What are the relevant tickets?

Fixes #1426

What's this PR do?

Adds a new management command, refund_fulfilled_order, that will do a "dry refund" of a fulfilled order. That is, this will update the status of the order to "refunded" and will adjust the learner's enrollments, but will not attempt to refund the learner's money via CyberSource.

How should this be manually tested?

1. Complete a transaction in the system.
2. Run refund_fulfilled_order using the reference number. Specify each flag separately and check the outcome - if --audit is specified, the enrollment should be converted to an audit one, and if --unenroll is specified, the enrollment should be unenrolled.

[codecov-commenter]

Codecov Report

Merging #1451 (4b493d9) into main (acd0936) will increase coverage by 1.63%.
The diff coverage is 0.00%.

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@@            Coverage Diff             @@
##             main    #1451      +/-   ##
==========================================
+ Coverage   81.31%   82.95%   +1.63%     
==========================================
  Files         164      311     +147     
  Lines        8971    13339    +4368     
  Branches      946      949       +3     
==========================================
+ Hits         7295    11065    +3770     
- Misses       1404     2003     +599     
+ Partials      272      271       -1     

Impacted Files	Coverage Δ
...erce/management/commands/refund_fulfilled_order.py	0.00% <0.00%> (ø)
ecommerce/models.py	85.31% <0.00%> (ø)
frontend/public/src/reducers/notifications_test.js	100.00% <0.00%> (ø)
...end/public/src/components/forms/ApplyCouponForm.js	100.00% <0.00%> (ø)
frontend/public/src/components/OrderSummaryCard.js	10.25% <0.00%> (ø)
frontend/public/src/lib/notificationsApi_test.js	100.00% <0.00%> (ø)
.../public/src/components/forms/ChangePasswordForm.js	100.00% <0.00%> (ø)
...ublic/src/components/forms/ChangeEmailForm_test.js	100.00% <0.00%> (ø)
frontend/public/src/lib/queries/users_test.js	100.00% <0.00%> (ø)
frontend/public/src/components/UserMenu_test.js	100.00% <0.00%> (ø)
... and 139 more

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[annagav]

LGTM 👍