Add a lock icon to confidential issues on search results

app/views/search/results/_issue.html.haml

 .search-result-row
   %h4
+    - if issue.confidential?
+      = icon('lock')
     = link_to [issue.project.namespace.becomes(Namespace), issue.project, issue] do
       %span.term.str-truncated= issue.title
     .pull-right ##{issue.iid}