Update snippets UI

What does this MR do?

Updates snippets UI (mainly mobile view) to new UI proposed in #18849 (closed).

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

Does anyone know the implementation difference between app/views/snippets/show.html.haml and app/views/projects/snippets/show.html.haml? I have only been updating the latter as it's the only one I can find implemented. I assume the former is rendered somewhere seeing as there is controllers for it too. But this doesn't seem very DRY from my current perspective.

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

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

What are the relevant issue numbers?

Closes #18849 (closed)