Resolve "Refactor code quality similar to JUnit tests"

What does this MR do?

• ee/app/serializers/ee/merge_request_widget_entity.rb - expose the artifacts file using the new architecture, make it generic

What are the relevant issue numbers?

Closes #7125 (closed)
Related https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20390
Related https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21892
CE MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21932

Does this MR meet the acceptance criteria?

• N/A Changelog entry added, if necessary
• N/A Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• N/A Conforms to the database guides
• EE specific content should be in the top level /ee folder
• N/A For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?