Allow admins to retrieve user agent details for an issue or snippet
Adds API endpoints to retrieve user agent details to all spammable models.
Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/29508
- Changelog entry added, if necessary
- Documentation created/updated
- API support added
- Tests
- Added for this feature/bug
- All builds are passing
- Conform by the merge request performance guides
- Conform by the style guides
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Branch has no merge conflicts with
master(if it does - rebase it please) - Squashed related commits together