Merge branch '19692-gray-block' into 'master'

Update header block class on snippets page

## What does this MR do?
Changes gray block in `Snippets` to new white sub header style

## What are the relevant issue numbers?
Closes #19692 

## Screenshots (if relevant)
![Screen_Shot_2016-07-11_at_4.17.10_PM](/uploads/f4cea2d019d30fc7c6adc7e3b26590b4/Screen_Shot_2016-07-11_at_4.17.10_PM.png)

See merge request !5192

app/views/projects/snippets/index.html.haml

 - page_title "Snippets"
 
-.row-content-block.top-block
+.sub-header-block
   .pull-right
     - if can?(current_user, :create_project_snippet, @project)
       = link_to new_namespace_project_snippet_path(@project.namespace, @project), class: "btn btn-new", title: "New Snippet" do