Merge branch 'prefer-to_reference' into 'master'

Use `Snippet#to_reference` directly

## What does this MR do?

Instead of hard coding snippet reference, we could just use the existing method for that.

## Why was this MR needed?

To address https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4338#note_12166325 from @DouweM

/cc @jschatz1 

See merge request !4379

app/views/shared/snippets/_header.html.haml

@@ -4,7 +4,7 @@
       = visibility_level_label(@snippet.visibility_level)
     = visibility_level_icon(@snippet.visibility_level, fw: false)
   %strong.item-title
-    Snippet $#{@snippet.id}
+    Snippet #{@snippet.to_reference}
   %span.creator
     created by #{link_to_member(@project, @snippet.author, size: 24, author_class: "author item-title")}
     = time_ago_with_tooltip(@snippet.created_at, placement: 'bottom', html_class: 'snippet_updated_ago')