Fixes issue with markdown snippet not being copyable

On markdown snippets this adds a hidden div with the raw markdown content so that it can be copied

Closes #13882

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

 - unless @snippet.content.empty?
   - if markup?(@snippet.file_name)
+    .hidden.blob-content{data: {blob_id: @snippet.id}}
+      = @snippet.data
     .file-content.wiki
       = render_markup(@snippet.file_name, @snippet.data)
   - else