Fix emoji paths in relative root configurations

What does this MR do?

If a site specifies a relative URL root, emoji files would omit the path from the URL, leading to lots of 404s.

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

At first, I tried to use ActionView::Helpers::AssetUrlHelper.asset_url since this is what it's intended to do. But this helper function is extremely slow, and it took minutes to generate the URLs for the hundreds of links needed for each emoji.

Why was this MR needed?

Because emojis were broken in relative URL installations

What are the relevant issue numbers?

#15642 (closed)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together