Merge branch 'copy-markdown-snippet' into 'master'

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

See merge request !3153

app/assets/stylesheets/pages/snippets.scss

@@ -28,3 +28,11 @@
   border: 1px solid;
   line-height: 32px;
 }
+
+.markdown-snippet-copy {
+  position: fixed;
+  top: -10px;
+  left: -10px;
+  max-height: 0;
+  max-width: 0;
+}

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

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