Add csv export button to domain list in registrar area

#248

app/views/registrar/domains/index.haml

@@ -50,6 +50,7 @@
              
             %span.glyphicon.glyphicon-search
              
+          = button_tag t('.export_csv_btn'), id: 'export-domains-csv-btn'
           %button.btn.btn-default.js-reset-form
             = t(:clear_fields)
 %hr

spec/features/registrar/domains/csv_export_spec.rb

@@ -2,7 +2,17 @@ require 'rails_helper'
 
 RSpec.feature 'CSV Export', db: true do
   scenario 'csv file download' do
+    sign_in
     visit registrar_domains_path
-    click_on 'download-domains-csv-btn'
+    click_on 'export-domains-csv-btn'
+  end
+
+  def sign_in(user: create(:user_with_valid_password))
+    visit registrar_login_path
+
+    fill_in 'depp_user_tag', with: user.email
+    fill_in 'depp_user_password', with: user.password
+
+    click_button 'Login'
   end
 end