Use the icon helper at wikis/_new.html.haml

app/views/projects/wikis/_new.html.haml

@@ -14,8 +14,8 @@
                 %span Page slug
               = text_field_tag :new_wiki_path, nil, placeholder: 'how-to-setup', class: 'form-control', required: true, :'data-wikis-path' => namespace_project_wikis_path(@project.namespace, @project), autofocus: true
               %span.new-wiki-page-slug-tip
-                %i.fa.fa-lightbulb-o
-                 Tip: You can specify the full path for the new file.
+                =icon('lightbulb-o')
+                Tip: You can specify the full path for the new file.
                 We will automatically create any missing directories.
             .form-actions
               = button_tag 'Create Page', class: 'build-new-wiki btn btn-create'