Create a request callback method for dj-stripe

[JacquelineMorrissette]

Checklist

1. If you've added code that should be tested, add tests
2. If you've changed APIs, update (or create!) the documentation
3. Ensure the tests pass
4. Make sure that your code lints and that you've followed our coding style
5. Write a description of your work suitable for publishing on our forum
6. Mention any related issues in this repository (as #ISSUE) and in other repositories (as kobotoolbox/other#ISSUE)
7. Open an issue in the docs if there are UI/UX changes

Description

Create a callback function for dj-stripe to use a custom subscriber model

Related issues

Other Notes

The need for a callback function is supposed to be depreciated according to the stripe documentation, but errors are thrown when using the djstripe_sync_models management command. The function can do absolutely nothing, it just needs to exist

[bufke]

Consider this approved and good to merge once you review my comments. I don't need to re-review. Thanks.