* refactor: remove resume url checks for performance
Existing "get resume urls" loads the course structure to verify the last
completed block actually exists. This is a potentially very expensive
operation, especially for users with many enrollments. This isn't a huge
issue since Learning MFE already has a fallback behavior for this case.
* refactor: remove now-unnecessary serializer check
Old behavior passed an empty string for missing resume URL and
serializer would cast to None. Instead, update simply passes through
None for missing resume url
* style: update dependency ordering
* style: run black on some more files
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