Convert admin/registrations datatable to AJAX-backed

Supercedes https://github.com/openSUSE/osem/pull/2031 .

Note:

* "Questions" modals have been dropped from this view; questions are no
  longer asked on registration, since Surveys(PR#1100)  were added.

app/controllers/admin/registrations_controller.rb

@@ -15,6 +15,13 @@ module Admin
       @registration_distribution = @conference.registration_distribution
       @affiliation_distribution = @conference.affiliation_distribution
       @code_of_conduct = @conference.code_of_conduct.present?
+
+      respond_to do |format|
+        format.html
+        format.json do
+          render json: RegistrationDatatable.new(view_context, conference: @conference)
+        end
+      end
     end
 
     def edit; end