Resolve "Test compiling frontend assets for production in CI"

What does this MR do?

Adds a new rake gitlab:assets:compile step to GitLab's CI config which tests compiling production assets and runs webpack-bundle-analyzer on the result. Bundle analyzer results are saved to webpack-report directory and published to gitlab pages.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

We weren't testing our production asset compilation step outside of omnibus-gitlab so errors in our webpack.config.js file or our package.json could go unnoticed for a long time.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #28450 (closed)