BitBucket Server Importer - Preserve PR (MR) reviewers
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### Release notes
Until now, BitBucket Importer did not import Pull Request reviewers and instead categorised them as Participants. Information on Pull Request reviewers is important to be preserved from an audit and compliance perspective. In 16.3 we are adding support for correctly importing PR reviewers from BitBucket, that in GitLab become Merge Request reviewers.
### Proposal
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Currently, BitBucket Importer is not importing PR reviewers. Reviewers, after import, are categorised as Participants, please see example:


The expectation is that this information should be preserved from an audit and compliance perspective. Reviewers for each PR are returned by the BB API whose details are captured within a ‘user’ object, and this same ‘user’ object is used by the author/creator.
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