Add classes externally

app/views/projects/protected_branches/_create_protected_branch.html.haml

-= form_for [@project.namespace.becomes(Namespace), @project, @protected_branch] do |f|
+= form_for [@project.namespace.becomes(Namespace), @project, @protected_branch], html: { class: 'new-protected-branch js-new-protected-branch' } do |f|
   .panel.panel-default
     .panel-heading
       %h3.panel-title