Add csv export button do domain list in registrar area

#248

spec/features/registrar/domains/csv_export_spec.rb

@@ -1,18 +1,12 @@
 require 'rails_helper'
 
 RSpec.feature 'CSV Export', db: true do
+  background do
+    sign_in(user: FactoryGirl.create(:api_user))
+  end
+
   scenario 'csv file download' do
-    sign_in
     visit registrar_domains_path
     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