Refactor bitbucket server representations to refer to actor in consistent manner
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The following discussion from !171835 should be addressed:
- [ ] @jnutt started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/171835#note_2197071849): (+1 comment)
> **thought:** I wonder if we can consolidate these `approver`/`decliner` under something generic like `actor`.
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