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Was the change discussed in an issue or in the forum before?
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Link issues and relevant forum posts here.

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Checklist
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- [ ] I have added tests for all changes in this PR
- [ ] I have added documentation for the changes (in the manual)
- [ ] There's a new file in `changelog/unreleased/` that describes the changes for our users (template [here](https://github.com/restic/rest-server/blob/master/changelog/TEMPLATE))
- [ ] I have run `gofmt` on the code in all commits
- [ ] All commit messages are formatted in the same style as [the other commits in the repo](https://github.com/restic/rest-server/commits/master)
- [ ] I'm done, this Pull Request is ready for review

Update PR template to encourage preceding issues.
Update PR template to encourage preceding issues.