CI artifacts:reports should be available with GitHub PR integration (and other external repositories)
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### Problem to solve
When using GitLab Merge Requests, [reports](https://docs.gitlab.com/ee//ci/yaml/README.html#artifactsreports) such as tests (e.g., [`artifacts:reports:junit`](https://docs.gitlab.com/ee//ci/yaml/README.html#artifactsreportsjunit)) and performance ([`artifacts:reports:performance`](https://docs.gitlab.com/ee//ci/yaml/README.html#artifactsreportsperformance-premium)) are posted to the Merge Request, but are not visible from the CI page (see, e.g., gitlab-ce#49212). When using [GitLab CI/CD with GitHub](https://about.gitlab.com/solutions/github/), there is no GitLab Merge Request and this information is not posted to the GitHub Pull Request. This severely limits the audience of GitLab CI/CD for projects hosted at GitHub, and since this limitation isn't documented, gives a poor impression of GitLab CI/CD relative to other services.
### Intended users
Anyone who uses [GitLab CI/CD with GitHub](https://about.gitlab.com/solutions/github/) or other [CI/CD for external repositories](https://docs.gitlab.com/ee/ci/ci_cd_for_external_repos/).
### Further details
This is a standard feature offered by all other CI/CD platforms, including GitLab CI/CD for GitLab repositories.
### Proposal
* Make all `artifacts:reports` reachable from the CI interface; see gitlab-ce#49212.
* Add reports to external pull requests (GitHub, Bitbucket) to the extent they support doing so.
### Permissions and Security
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### Documentation
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### Testing
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### What does success look like, and how can we measure that?
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### Links / references
issue