Fix BaseRepositorySubForm to use new name for adding field prefixes.
Review Request #2204 — Created Aug. 9, 2021 and updated
In 1.11, Django renamed `Form.addprefix` to `Form.add_prefix`. When we updated to it, we never made the change, and so the repository forms were still calling the old function. This didn't affect any of the built-in SCMTools, but for third-party tools that want to prefix keys into the extra_data, it was failing. Testing Done: Used in conjunction with an external SCMTool. Reviewed at https://reviews.reviewboard.org/r/11719/