Move artifact management app data to helper

What does this MR do and why?

For #388041 (closed)

This MR moves the data for the artifacts page out of the haml template and into a new artifacts_helper.

Why? It's easier to read and easier to test; and as the artifacts page grows, the data it needs is likely to grow more complex, so we should do this now to make future changes easier too.

Screenshots or screen recordings

Purely a code refactor - no UI changes are expected.

How to set up and validate locally

1. git checkout 388041-move-artifacts-haml-data-to-helper
2. echo "Feature.enable(:artifacts_management_page)" | bundle exec rails console
3. navigate to a project, run a pipeline that generates artifacts
4. in the sidebar select CI/CD => Artifacts
5. if you see artifacts data on the page (if the page appears the same as master with the feature flag enabled), then it worked!

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.