All content finder UI: Persist feature flag query param

This will make it easier for the team to test the new UI in production
by persisting the feature flag query parameter across form submissions.

We can remove this once the new UI is live.

app/views/finders/show_all_content_finder.html.erb

@@ -15,6 +15,7 @@
 
 <form method="get" action="<%= content_item.base_path %>" data-ga4-change-category="clear-all-filters">
   <%= hidden_field_tag :parent, @parent if @parent.present? %>
+  <%= hidden_field_tag :enable_new_all_content_finder_ui, params[:enable_new_all_content_finder_ui] if params[:enable_new_all_content_finder_ui].present? %>
 
   <div class="govuk-width-container">
     <%= render 'govuk_publishing_components/components/contextual_breadcrumbs', content_item: content_item.as_hash %>